Aim: Prader-Willi syndrome (PWS) is a genetic disorder historically characterized by two phenotypic stages. The early phenotype in infants is associated with hypotonia, poor suck, and failure to thrive. In later childhood, PWS is associated with intellectual disability, hyperphagia, as well as growth and sex hormone deficiency. Little is known about the transition between phenotypes. This study investigates the nature of the change in infancy and childhood PWS. Method: Forty-six children (22 females, 24 males; mean age 2y 9mo, SD 18.9mo; range 7mo-5y) with genetically confirmed PWS participated. Information was obtained on childhood height and weight, and eating behaviour from case notes and by parental interview. Results: Weight standard deviation scores (SDS) started to exceed height by the end of the first year. Height SDS appeared to fall from near normal at birth until stabilizing below normal around 2 years. Half of the children whose body mass index (BMI) was higher than normal at interview had food interests greater than that of their peers, and the age at which increased age-appropriate eating was first noted was later than the increase of BMI SDS. Interpretation: Obesity may develop before the increased interest in food, suggesting underlying physiological factors independent of appetite control may be important.