The microscopic development of the normal lambdoid suture was studied in autopsy specimens from 19 normal subjects ranging in age from 20 weeks' gestation to 60 years. The cellular activity at the suture varied considerably with age; however, maximal activity was seen in specimens approximately 3 months of age. There were several unusual features, including a high incidence of cartilaginous differentiation and the presence of intrasutural Wormian bones. Forty-one specimens from 37 patients with isolated lambdoid synostosis were also studied pathologically. Only three cases showed bone union across the suture, which appears to be a result of closure rather than fusion as in other synostoses. The remainder of the cases showed varying degrees of increased cellular proliferation at the suture line, resulting in exaggerated and prolonged sutural activity. Morphologically, this produced increased interdigitation and fibrous adhesion between the suture margins.