By a systemic search of osteopetrosis in the county of Funen, Denmark, the prevalence was 5.5/100,000 inhabitants. The study disclosed 33 patients of whom 32 had the mild, autosomal dominant form of osteopetrosis. Two obligate carriers, who had the genotype but were not phenotypically affected, were disclosed. There was a great variation in the clinical manifestations; 39% were asymptomatic. The age of first appearance of symptoms also varied widely (8-76 years), with a tendency to increasing symptoms with aging. The frequency of fractures was low. Plasma inorganic phosphate was low in 7% of the patients, and plasma acid phosphatase was increased in 39%.