The development and profiles of adaptive and maladaptive behavior of 21 adolescents and adults with Prader-Willi syndrome were cross-sectionally examined with the Vineland Adaptive Behavior Scales and Achenbach's Child Behavior Checklist (CBCL). Adaptive strengths emerged for the group as a whole in daily living skills, and this strength became more pronounced with increasing age. A relative weakness was found in socialization, most notably in coping skills. CBCL findings indicated that externalizing behaviors were particularly heightened in adolescence and that many behaviors previously described as either emerging or worsening in adolescence also persist into the adult years (e.g., temper tantrums, arguing, irritability, stubbornness, lying, skin picking, obsessions, defiance). Certain elevated CBCL behaviors were unique to young versus old age groups, and aging in this syndrome may be associated with heightened confusion, withdrawal, and fatigue. The need to study adaptive and maladaptive features in a wider age range of subjects with Prader-Willi syndrome was emphasized.