We achieved $1.82 in adjusted earnings per share, a 7% increase over Q1 of 2020.
Adjusted EBITDA increased 3% to $104 million.
As for our growth metrics, as expected, the average number of paid worksite employees in Q1 of 2021 declined by 2% compared to Q1 of 2020 and included the loss of one large enterprise account that we referred to in our previous earnings call.
It's also important to note that during the challenges of the pandemic over the past year, we've increased the number of clients by 8%.
Now, as most of you are aware, the year-end transition from 2020 to 2021 in which we enroll new clients from our fall sales campaign and renew approximately 45% of our existing clients is important to our 2021 starting point and, therefore, our full year growth expectations.
Worksite employees paid from new client sales were in line with our budget and we're 93% of Q1 of 2020, a period prior to the onset of the pandemic.
Excluding this one account, attrition totaled 9%, an improvement over Q1 of 2020's attrition of 11%.
Now let's move on to gross profit, which increased by 7% over Q1 of 2020 on the 2% decline in worksite employees.
In addition, the Q1 upside resulted from lower on -- Q1 upside resulting from the lower SUTA rates during the quarter, we received a $6 million federal payroll tax refund related to the prior year.
We continue to grow our sales force at targeted levels with a 7% increase in the average number of trained business performance advisors.
In total, operating expenses increased 13% over Q1 of 2020, however were flat when excluding performance-based compensation.
During the quarter, we repurchased 340,000 shares of stock at a cost of $30 million, paid out $15 million in cash dividends and invested $12 million in capital expenditures.
We ended Q1 with $197 million of adjusted cash and $370 million of debt.
This quarter, our paid worksite employees from prior bookings reflected our solid fall campaign sales and came in at 93% at the same period in 2020, which was largely pre-pandemic.
Our sales team is off to an impressive start to the year achieving 102% of our budgeted bookings in this quarter.
The number of trained business performance advisors was up 7% and this team increased discovery calls by 16% and business profiles by 21%.
The number of new clients sold also increased 16% over the same period last year, which is notable since most of Q1 last year was pre-pandemic.
However, the pipeline is rebuilding rapidly with a 27% increase in leads and a 13% increase in proposal opportunities over last year.
WX proposals increased 90% over the same period last year and book sales more than doubled in both the number of accounts and employees sold.
Our WX initiative is an important long-term plan to increase sales efficiency, providing a traditional employment HR bundle alternative at a lower price point is designed to capitalize on the investment we've already made in our team of more than 650 BPAs across the country that are calling on more than 40,000 small businesses each year.
Our goal over time is to convert some portion of the nine out of 10 prospects that we do not sell WO into WX clients and ultimately upgrade them to WO, increasing our sales efficiency.
Our workforce optimization client retention was also a highlight this quarter, improving by 15% over last year, excluding the large client loss discussed last quarter.
As we entered the new year, our average size client was down approximately 8% in the number of worksite employees after trimming back during the pandemic.
When asked how optimistic you are with the outlook for your business this year, 86% were very or somewhat optimistic compared to 48% late last year and 72% in late 2019.
Further, 81% of those surveyed expect organizational performance to be better than last year and 53% expect to add employees and 35% expect to increase compensation.
Only 3% expect to reduce staff and only 1% expect to decrease compensation.
In fact, when asked about last year's results, 71% said they were better or as expected and only 10% said their results were worse than expected, which we believe reflects the quality of our client base and the success of our strategy to target the best, small and mid-sized businesses.
Most notable this quarter was commission up over 11% from the same period last year, a double-digit increase for the second consecutive quarter.
We generally see when commissions are up over 6% from the prior year, hiring and compensation increases subsequently trend upwards.
Current trends in sales retention and hiring in the client base, combined with the comparison to Q2 2020 shutdown-related layoffs, has us on track to move from minus 2% year-over-year growth in the first quarter to 5% to 6% growth in the second quarter.
We are now forecasting 4% to 6% worksite employee growth for the full year, an improvement over our initial guidance of 2% to 6% growth.
We are forecasting Q2 paid worksite employee growth of 5% to 6% over Q2 of 2020, a period which was significantly impacted by the onset of the pandemic.
So when taking into account these factors, we are forecasting adjusted EBITDA in a range of $250 million to $280 million, up from our initial guidance of $225 million to $275 million.
As for full year 2021 adjusted EPS, we are now forecasting a range of $3.83 to $4.40, up from our previous guidance of $3.27 to $4.20.
As for Q2, we are forecasting adjusted EBITDA in a range of $44 million to $49 million and adjusted earnings per share from $0.60 to $0.70.

Summary:
We achieved $1.82 in adjusted earnings per share, a 7% increase over Q1 of 2020.
As for full year 2021 adjusted EPS, we are now forecasting a range of $3.83 to $4.40, up from our previous guidance of $3.27 to $4.20.
As for Q2, we are forecasting adjusted EBITDA in a range of $44 million to $49 million and adjusted earnings per share from $0.60 to $0.70.