We report the first prospective longitudinal study of dietary intake, weight, height, and skinfold measurements during the early phase of four Prader-Labhart-Willi syndrome (PLWS) individuals (two males and two females). Although caloric intake ranged from 80 to 90 percent of recommended daily allowance during our study of the four PLWS infants, obesity still occurred. Our findings suggest that the onset of obesity in PLWS individuals occurs earlier than previously thought in spite of reduced caloric intake. The infants in our study reached the obese range judged by skinfold measurements greater than the 85th centile at an early age and before they were considered heavy based on weight for height criterion. We propose that skinfold measurements should be obtained on all individuals with PLWS and obesity judged by this criterion.