Four features of maladaptive behavior were examined in 25 children and 61 adults with Prader-Willi syndrome: age, gender, IQ, and the Body Mass Index (BMI). Among children, older subjects showed increased symptomatology relative to younger subjects, especially in depression and withdrawal. Among adults, maladaptive behaviors seem to show both steady and variable expressions, waxing and waning over time. Boys showed heightened depression relative to girls, findings which were not seen in the adults. No maladaptive behavior differences were found in high vs. low IQ subjects. Thinner adults with lower BMIs had higher maladaptive behavior scores relative to heavier subjects, particularly in internal states involving distressful affect and problems with thinking. Several explanations of these counterintuitive BMI findings are discussed, as is the need for prospective research on maladaptive behavior in children and adults with this syndrome.