OBJECTIVE: To compare the clinical features and T-cell subsets among 160 patients with acute HIV-1 infection not treated with antiretroviral agents from three different locations (Geneva, Seattle and Sydney). DESIGN: Patients with documented acute HIV-1 infection were enrolled in four prospective studies: one randomized placebo controlled trial (RCT) and three observational cohort studies. SETTING: All patients were diagnosed and followed in three university affiliated tertiary clinical care centers. METHODS: The chi-square test was used for comparing proportions and one way analysis of variance (ANOVA) for comparing continuous variables among these groups. Multiple regression analysis was used to identify the variables independently associated with the duration of acute HIV-1 infection. The differences in mean CD4 and CD8 load between centers were assessed using the random-effect models for the longitudinal data. RESULTS: Minor differences were noted in the frequency of symptoms among subjects enrolled at different locations. RCT patients reported a longer duration of symptoms (31 days) compared with that for observational patients (15 days; P < 0.0001). For the most common symptoms such as fever, skin rash, arthralgia, myalgia, and headaches, a longer duration was observed in the RCT group compared with that for observational patients (P range, 0.001 to < 0.0001). T-cells subsets within 100 days of seroconversion did not statistically differ by centre or by mode of recruitment. CONCLUSIONS: These results suggest a selection bias toward patients with longer symptomatic acute HIV-1 infection enrolled in the RCT. Data collected from RCT are not comparable to that collected in observational studies. However, data from collaborative international studies can be combined.