The natural history of Williams syndrome, including medical complications, growth patterns, and problems in adulthood, was investigated. A growth pattern characterized by delay in the first 4 years of life, catch-up growth in childhood, and low ultimate adult height was found. Despite multiple medical problems in infancy, including feeding problems, failure to thrive, colic, and otitis media, mean age at diagnosis was 6.4 years. Developmental disabilities and cardiovascular disease were the major concerns in childhood. The older children developed progressive joint limitation and hypertonia. Adult patients were handicapped by their developmental disabilities. Hypertension, and gastrointestinal and genitourinary problems occurred frequently. Independent living and competitive employment were limited less by the individual's physical problems than by the psychologic and adaptive limitations. Williams syndrome is a progressive disorder with multisystem involvement.