To assess the role of genetic factors in systemic lupus erythematosus (SLE), 12 twon pairs (seven definitely monozygotic, three definitely dizygotic) of which one or both twins had SLE, were studied and compared to 17 twin pairs (12 definitely monozygotic) previously described. In the present series, four of seven (57 per cent) definitely monozygotic pairs were clinically concordant for SLE, satisfying the preliminary criteria of the American Rheumatism Association (ARA). Concordance for the presence of antinuclear factor (ANF) and hypergammaglobulinemia was 71 and tinuclear factor (ANF) and hypergammaglobulinemia was 71 and 87 per cent, respiectively. These data closely agree with those on the 12 definitely monozygotic sets previously described. All three of the dizygotic sets in the present series were discordant for clinical SLE, although one clinically well twin had marked serologic abnormalities. Comparison of these data with thos from other first degree relatives of out twins clearly suggests a strong genetic component in the pathogenesis of SLE. The relative contribution of nongenetic and environmental factors to the expression of the disease is discussed.