Population and total hip replacement surveys show that primary osteoarthritis of the hip is uncommon in African Americans and rare in Asians, suggesting a genetic basis for this disease. We studied genetic influences on primary osteoarthritis of the hip by estimating monozygotic (MZ) and dizygotic (DZ) twin correlations using a two-stage data collection. A total of 6419 male veteran twins of the NAS-NRC Twin Registry, born between 1917 and 1927, were contacted by telephone (first stage). Telephone interview determined that 2% reported a total hip replacement for arthritis rather than fracture. X-rays of twin pairs in which one twin had undergone total hip replacement were sought and reviewed (second stage), and concordance for primary arthritis was determined based on x-ray diagnosis. Heritability of primary osteoarthritis, Kellgren & Lawrence Grade II and higher, was estimated using a covariance structure analysis for 2-stage data. The best-fitting model included only components for additive genetics and for unique environment. Additive genetics accounted for 53% (95% confidence interval 30-72%) in the liability for self reported hip replacement surgery and unique environment for the remaining 47% (95% confidence interval 28-70%). Additive genetics accounted for 61% (95% confidence interval 18-86%) of the variance in liability for x-ray determined primary osteoarthritis with unique environment accounting for the remaining 39%. These data establish a genetic influence on primary osteoarthritis of the hip in male twins and suggest that further work is indicated to isolate the genes responsible for this disease.