The social-emotional functions associated with Machiavellianism have been widely examined. However, most research on Machiavellianism is cross-sectional design, and has been conducted in adult populations. Using a sample of adolescents (n = 454), the current study employed a longitudinal design to examine how Machiavellianism relates to internalizing and externalizing behavior problems. The results indicated that (a) a significant increase in Machiavellianism scores and a significant decrease in difficulties scores were observed over the course of a year; (b) Machiavellianism shows significant positive associations, both cross-sectional and longitudinal, with nearly all categories of internalizing and externalizing behavior problems; (c) time 1 hyperactivity was a significant predictor of time 2 (one year later) Machiavellianism; and (d) over the course of a year, Machiavellianism still has a significant impact on subsequent conduct problems. The implications of these findings are interpreted and discussed.