['The 3-minute step test is a simple test of exercise capacity for children with cystic fibrosis (CF), but no data have been reported regarding its usefulness in adults or its prognostic value.', 'To determine the feasibility and acceptability of the 3-minute step test as a test of exercise capacity in adults with CF and whether test performance is associated with 12-month clinical outcomes.', 'From our out-patient clinic we prospectively recruited consecutive adult patients with CF in stable health. The 3-minute step test was conducted with a standardized protocol that included a 15-cm high step and external pacing at 30 steps/min. We measured heart rate, dyspnea, and S(pO(2)). With multiple linear regression analyses we assessed the relationships between step-test performance and change in FEV(1) and hospital days at 12 months.', 'The participants were 101 adults: 56 male, mean ± SD age 29 ± 9 years, percent-of-predicted FEV(1) 61 ± 23%, body mass index 22 ± 4 kg/m(2). Only 42% of the participants with mild CF achieved 70% of the predicted maximum heart rate during the 3-minute step test, compared to 77% of those with FEV(1) < 60% of predicted. The 22 patients who desaturated to < 90% during the 3-minute step test had a larger number of hospital days over the following 12 months than did those who did not (median 28 d vs 11 d, P < .001). Those who desaturated also had a greater FEV(1) decline (mean difference -117 mL, 95% CI -215 to -19 mL). Desaturation during the 3-minute step test was an independent predictor of both FEV(1) decline and days spent in hospital.'] Does desaturation during the 3-minute step test predict impaired 12-month outcomes in adult patients with cystic fibrosis?