The 6-min walk test (6MWT) is increasingly being used as a measure of 'functional ability' in young people with cerebral palsy, despite a lack of published evidence that it is reliable for this population. This study aimed to determine the test-retest reliability of the 6MWT for ambulant 11-17-year-old adolescents with cerebral palsy. Adolescents with cerebral palsy were invited to participate in a single testing session. They performed the 6MWT twice under controlled conditions (standardized 10 m course layout, standardized instructions and encouragement), with a 30-min rest. Forty-one participants (response rate 36.6%) were recruited (26 males, 15 females; mean age 13.6+/-1.6 years). No significant difference was found between the 6-min walk distance in trials 1 and 2 for the whole group (448.7+/-96.9 m vs. 449.5+/-102.1 m; P=0.81) or by sex and gross motor function subsets. The test-retest intraclass correlation coefficient was 0.98. Bland-Altman analyses revealed a bias of only -0.9 m, and that in 95% of cases, the second 6-min walk distance would fall within +/-43.1 m of the first 6-min walk distance. All participants successfully managed the testing procedure, appeared to understand what was expected of them, and experienced no ill effects from the 6MWT. These results indicate that, when carried out according to American Thoracic Society guidelines, the 6MWT is a reliable test for young ambulant people with cerebral palsy.