Preschool boys identified by their parents and/or teachers as active, inattentive, and impulsive (N = 69) at age 4 were compared with boys without problems (N = 43) on observational measures of symptom-related behaviors, obtained across contexts (home, laboratory, and preschool). Problem boys differed from comparison boys on measures of activity/inattention, noncompliance, and impulsivity obtained in the 3 settings. At a 2-year follow-up, when they were 6 years old, problem boys continued to differ from comparison boys on laboratory measures of activity and impulsivity; there also was some stability in these symptomatic behaviors. Implications of these findings for the emergence and persistence of externalizing problems in preschool boys are addressed.