In this experimental study, it was examined to what extent peers and sex were important predictors of risk taking behavior of adolescents. Participants were 140 Dutch adolescents (52.9% boys, 12-15 years) who completed the Balloon Analogue Risk Task (BART) as a measure of risk taking behavior, either individually or in the presence of homogenous or heterogeneous peer groups. Results showed that (a) adolescents took significantly more risk when they completed the BART with peers than when they completed the risk taking task individually, (b) boys took significantly more risk when they completed the task with peers than girls but not when they completed the task individually, and (c) boys in "boy-only triads" revealed the strongest risk taking behavior compared with "mixed-girl triads" or "girl-only triads." These results suggest that boys appear to be more susceptible to the influence of peers on risk taking behavior than girls.