This study examines family and child characteristics, parent and peer relationships, and early adolescent behavior as statistical predictors of trajectories of number of sexual partners from midadolescence through early adulthood using data from 527 participants in the Child Development Project. Early adolescent developmental antecedents accounted for modest variance in number of sexual partners. Latent growth models revealed that African American race, more advanced pubertal development, lower parental monitoring knowledge, association with more deviant peers, and lower grade point average in early adolescence each predicted having more sexual partners at age 16. In addition, non-African American race, lower child IQ, higher parental monitoring knowledge, and lower early adolescent internalizing problems each was associated with a higher rate of growth in number of sexual partners over time at the ages following 16. Latent growth mixture modeling identified subgroups with distinct trajectories of involvement with sexual partners that were associated with family and child characteristics, parent and peer relationships, and behavior in early adolescence.