Although many examples of familial SLE have been reported, the relative importance of genetic and environmental factors remains unclear. In an effort to better understand these factors, eight such families were studied. These families, plus one deceased pair and 31 described in the literature consisted of 8 pairs of identical twins, one pair of non-identical twins, and 16 sibling and 15 parent-offspring combinations. Fifty-three cases in 25 families provided sufficient documentation for detailed analysis. Each familial case was compared to his affected relative utilizing 23 clinical and laboratory features of SLE. A control group of non-related SLE patients, each matched to a familial case for age, sex, race and disease duration, was similarly analyzed. Impressive concordance for disease expression was found between pairs of identical twins and between parent and offspring. No such concordance was found between siblings and controls. These findings support genetic influences in the expression of SLE in identical twins and parents and offspring pairs. Comparison of the frequencies of clinical and laboratory attributes in the familial group as opposed to non-familial groups showed no real differences. Thus, familial SLE is probably not a different disease entity from non-familial SLE. The finding of four father-offspring pairs lessens the possibility that SLE is transmitted during the perinatal period. The onset of SLE in each of identical twins occurred within an average of 2 years. In siblings, however, while the average difference in age at onset was 9 years, the average difference in time of onset (actual date) was only 3 years. Comparable figures for parents and offspring were 20 and 8 years, respectively. These data suggest environmental influences in the initiation of SLE, especially in siblings. Studies of 27 unaffected first-degree relatives in our eight families revealed two sisters with thyroid disease, persistent leukopenia and sedimentation rate elevation. They were daughters of a patient with SLE and Hashimoto's thyroiditis and sisters of a patient with SLE. The remaining 25 relatives were clinically, hematologically, biochemically, and serologically normal. While these studies suggest both genetic and environmental influences in the pathogenesis of SLE, further studies are necessary to elucidate the mechanisms.