The increasing prevalence of children's attention-deficit hyperactivity disorder (ADHD) and internalizing behaviors requires investigation into putative risk factors. This longitudinal project of 105 preschoolers (54.3% girls; 90% White) examined influences of early life experiences (perinatal stressors, home environment, maternal personality) and individual differences (child temperament and activity level) on problem behaviors. At age 4 years, activity levels were observed, and parents reported on temperament and home environment. At age 5 parents reported on child ADHD behaviors, internalizing problems, and maternal personality. ADHD behavior was predicted by age 4 temperamental activity and a negative home environment. A significant interaction between temperamental activity and home environment showed that greater age 4 temperamental activity significantly predicted fewer internalizing problem behaviors at age 5, but only for children in homes with low instability. Thus, greater activity levels may be risk factors for ADHD behaviors but protective for internalizing problems, underscoring the importance of considering children's temperament within the home environment.