Previous studies have investigated trajectories of cognitive, language and adaptive functioning in Williams syndrome (WS) but little is known about how other aspects of the Williams syndrome behavioural phenotype change across the life-span. Therefore, the present study examined age associated changes in a number of different domains of functioning in adults with WS. Semi-structured interviews and standardized assessments of physical health, educational and occupational levels, self-care and independence, social interactions, and behavioural difficulties were conducted with adults with WS and their parents/carers. Ninety-two individuals with genetic confirmation of WS (50 males, 42 females) participated in a cross-sectional study (age range 19-55 years; mean 32 years) and 43 individuals (21 males, 22 females) were involved in a longitudinal study as they had taken part in a previous study of adults with WS. Not all individuals in the longitudinal cohort had genetically confirmed WS. Cross-sectional analyses revealed no age related differences in the areas of functioning investigated. However, in the longitudinal sample, significant improvements were reported in physical health, self-care and occupational skills. Social skills and adaptive behaviours had generally improved while behavioural difficulties had declined. In both cross-sectional and longitudinal cohorts there were significant correlations between adaptive functioning and IQ. These findings are consistent in indicating no age related decline in social or adaptive functioning in adults with WS, at least up to the age of 50-55 years. (Contains 8 tables.)