Objective: To assess whether gait performance indoors can predict walking ability outdoors in persons with late effects of polio. Design: Descriptive analyses of a convenience sample. Setting: A university hospital rehabilitation medicine outpatient department. Participants: Sixty-three individuals with prior polio (32 men and 31 women, mean age 68 ± 6 years). Main outcome measures: The 6-Minute Walk Test (6MWT) and a 340-m-long distance were used to assess gait performance indoors and walking ability outdoors, respectively. The Borg Rating of Perceived Exertion (RPE) Scale was used to assess perceived exertion after the gait performance tests. Results: Significant correlations (p < 0.01) were found between the 6MWT and the outdoor walking with regard to gait speed (r = 0.92) and perceived exertion (rho = 0.68). A majority of the participants walked significantly (p < 0.001) faster outdoors than indoors; the average gait speed was 1.3 ± 0.3 m/s outdoors and 1.2 ± 0.3 m/s indoors. There was no significant difference in perceived exertion; median RPE was 13 (range 11–19) after the outdoor walking and 13 (range 9–17) after the 6MWT. Conclusion: The strong relationship between gait speed indoors and gait speed outdoors indicates that the 6MWT is a useful test for physiotherapists to predict walking ability in everyday life in ambulatory persons with mild to moderate late effects of polio.