OBJECTIVES: Although the 6-minute walk test (sub-maximal exercise test) historically has been used to evaluate the effect of pharmacologic intervention in adults with pulmonary arterial hypertension (PAH), it was not widely regarded as appropriate for children at all ages.  Thus, Peak VO2 (maximal exercise test) was used in a pediatric PAH trial (A1481131) to evaluate the effect of treatment.  As this is the first large controlled trial using the Peak VO2 endpoint in this population, we investigated its performance in terms of correlational analyses. METHODS: Relationships between change in Peak VO2 and other endpoints were evaluated using correlation coefficients and regression analyses with the data from the 16-week randomized, placebo-controlled, clinical trial of sildenafil assessing Peak VO2 at both baseline and week 16 in 106 evaluable pediatric patients.  A Bland-Altman plot was used to assess the reliability of screen and baseline visit data. RESULTS: The intraclass correlation was 0.79 for the screening and baseline visit Peak VO2 data. Additionally, agreement between these visits was supported by a Bland-Altman plot.  Correlations of percentage changes in Peak VO2 from baseline with the change from baseline in the Physician Global Assessment (overall and for sildenafil doses) and in World Health Organization Functional Class (WHO FC, with baseline of I and III/IV) correlated well (correlations of ≥0.40) and were responsive to change, while the low correlation with the Subject Global Assessment (0.12) suggested influence by factors associated with child and parental proxy response and instrument administration.  Percentage changes in Peak VO2 gave no real association (0.04) with the Family Cohesion of the Child Health Questionnaire. CONCLUSIONS: In pediatric PAH, Peak VO2 measurements exhibited good reliability. Improvements in Peak VO2 were shown to be associated with improvements in qualitative clinical endpoints, including WHO FC and physician global assessment.