To investigate the safety of the Chester step test (CST) and the modified incremental step test (MIST) to assess functional capacity in patients hospitalized with acute lung diseases.Seventy-seven hospitalized patients (46 men) performed the CST and the MIST on the same day in random order and the 6-minute walk test (6MWT) the following day. A control group of 20 healthy volunteers performed the same procedures.No adverse events were observed during the step tests. Mean exercise-induced desaturation was similar between the CST (-2%; 95% CI, -6 to 0), the MIST (-2%; 95% CI, -6 to -1), and the 6MWT (-2%; 95% CI, -5 to 0). Significant correlations were found between the number of steps on the CST and the MIST and lung function (forced vital capacity: r = 0.52 and r = 0.57, respectively), dyspnea (r =-0.54 and r =-0.41, respectively), and the distance walked on the 6MWT (r = 0.59 and r = 0.64, respectively). In addition, a significant correlation was observed between the number of steps and the length of hospitalization. Patients performed poorer on the step tests compared with healthy subjects (P < .05).The CST and the MIST are safe and can be used as an alternative test to assess functional capacity in patients hospitalized for acute lung diseases. The worse the performance on the step tests, the lower the pulmonary function and the distance walked on the 6MWT, the greater the dyspnea, and the longer the hospitalization.