First degree relatives and spouses of 36 patients with systemic lupus erythematosus (SLE) and 37 with discoid lupus erythematosus (LE) were assessed using the ARA criteria. They were compared with relatives and spouses of patients with other rheumatic and related complaints. Definite SLE was present in 3.9% of relatives of SLE probands, 2.6% of discoid relatives and 0.3% of controls. Discoid LE was diagnosed in 0.6% of SLE and 3.5% of discoid families compared with 0.5% of controls. None of the spouses of LE probands had SLE or discoid LE. The data gave the best fit for a polygenic inheritance with a heritability of 66 +/- 11% for SLE and 44 +/- 10% for discoid LE. Genetic factors are thus less important in SLE and discoid LE than in generalized osteoarthritis, spondylitis or gout with heritabilities of 90, 72 and 90%, respectively.