Prader-Willi syndrome (PWS) is a genetic disorder which has widespread developmental consequences including motor, cognitive and language delay. Previous research on PWS children has focused primarily on phonological development and dysfluency. In the present study, the lexical development of a boy with PWS was investigated in a series of 18 play sessions recorded over a 4 month period from the ages 3;7 to 3;11. In comparison to the language development of children with Down syndrome this child with PWS appears to display a distinct developmental pattern. The possibility of detailing a behavioural phenotype of genetic disorders affecting language development is discussed. (Contains 9 tables.)