The effects and predictors of duration in youth mentor relationships were examined. The study included 1,138 young, urban adolescents (Mean age 12.25), all of whom applied to Big Brothers Big Sisters programs. The adolescents were randomly assigned to either the treatment or control group, and administered questions at baseline and 18 months later Adolescents in relationships that lasted a year or longer reported the largest number of improvements, with progressively fewer effects emerging among youth who were in relationships that terminated earlier. Adolescents who were in relationships that terminated within a very short period of time reported decrements in several indicators of functioning. Older adolescents, as well as those who had been referred for services or had sustained emotional, sexual or physical abuse, were most likely to be in early terminating relationships, as were married volunteers aged 26-30 and those with lower incomes. Several dyadic factors were also found to be related to earlier terminations, including race, gender, and relationship quality.