We delivered exceptional results driven by differentiated commercial solutions with revenues of $52.5 billion for the first fiscal quarter, representing growth of 10% year-over-year and our adjusted earnings per share increasing 24% versus the prior year quarter.
One customer group, for whom we have consistently created new incremental value, is our independent pharmacy customers including our more than 5,000 Good Neighbor Pharmacy and Elevate Provider Network members.
These include ensuring that MWI associates are accessible to our customers 24/7 and bolstering our customers' abilities to offer virtual services to a pet caring clients including innovative client communication solutions and home delivery services of quality medications and pet care products.
Beginning with our first quarter results, we finished the quarter with adjusted diluted earnings per share of $2.18, an increase of 24%, primarily due to exceptional operating income growth across our businesses.
Our consolidated revenue was $52.5 billion, up 10%, driven by revenue growth in both the Pharmaceutical Distribution Services segment and Other, which includes our Global Commercialization Services & Animal Health businesses.
Gross profit increased 15% to $1.4 billion, driven by increases in gross profit in each operating segment.
In the quarter, gross profit margin increased 12 basis points from the prior year quarter.
The PharMEDium comparison and the inventory writedown reversal contributed one-third of the 12 basis point gross profit margin improvement.
Consolidated operating income was $617 million, up $122 million or 25% compared to the prior year quarter.
To support our revenue growth while protecting, supporting and appropriately compensating our frontline associates, operating expenses grew 8% to $810 million.
Operating expenses as a percent of revenue was 1.54% which is a 2 basis point decline from the prior year quarter.
Moving now to net interest expense, which increased $3 million to $34 million primarily due to a decrease in interest income resulting from a decline in investment interest rates.
Our effective tax rate was 22%, up from 21%, in the first quarter of fiscal 2020.
Our diluted share count declined modestly to 206.8 million shares.
Regarding free cash flow and cash balance, our adjusted free cash flow was $838 million in the first quarter.
We ended the quarter with $4.9 billion of cash of which $1.1 billion was held offshore.
Beginning with Pharmaceutical Distribution Services, segment revenue was $50.5 billion, up 10%, driven by increased specialty product sales, including COVID-19 therapies, as well as growth at some of our largest customers and broadly across our businesses.
Segment operating income increased about 27% to $496 million with operating income margin up 13 basis points.
As a reminder, the exit of the PharMEDium business represented a $20 million tailwind to the segment's operating income, roughly half of which is in gross profit and the other half in operating expense.
Excluding the PharMEDium tailwind, segment operating income growth would have been up 20%.
In the quarter, total revenue was $2.1 billion, up 11%, driven by growth across the three operating segments.
Operating income for the group was up $17 million or 16%, primarily due to growth at MWI and World Courier.
So I will now turn to our fiscal 2021 guidance.
Given the cash needs associated with the Alliance acquisition, we are narrowing our guidance from a range of 206 million to 207 million and we now expect to finish the year around 207 million shares outstanding.
All other financial guidance metrics for fiscal 2021 remain unchanged.

Summary:
We delivered exceptional results driven by differentiated commercial solutions with revenues of $52.5 billion for the first fiscal quarter, representing growth of 10% year-over-year and our adjusted earnings per share increasing 24% versus the prior year quarter.
Beginning with our first quarter results, we finished the quarter with adjusted diluted earnings per share of $2.18, an increase of 24%, primarily due to exceptional operating income growth across our businesses.
So I will now turn to our fiscal 2021 guidance.
All other financial guidance metrics for fiscal 2021 remain unchanged.