This study investigated interactions between observed temperamental effortful control and observed parenting in the prediction of externalizing problems. Child gender effects on these relations were examined. The relations were examined concurrently when the child was 3 years old and longitudinally at 4.5 years. The sample included 89 two-parent families and their firstborn children. Children with a low level of effortful control were most at risk of displaying externalizing problems. However, more parental positive control seemed to buffer this risk. Boys were at risk of displaying externalizing problems, but again this was buffered by parental positive control. Effortful control was more strongly related to concurrent externalizing problems in boys than in girls, but girls' effortful control had a greater long-term effect on externalizing problems. (Contains 3 figures and 2 tables.)