BACKGROUND: 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. OBJECTIVE: 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. METHODS: 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. RESULTS: 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. CONCLUSIONS: Desaturation during the 3-minute step test is associated with long-term pulmonary deterioration and more hospital days in adults with CF. The 3-minute step test may be a useful screening test for patients with moderate to severe CF lung disease, who require increased intervention and monitoring.