A retrospective study of 112 cases of lupus erythematosus, 103 acute disseminated lupus erythematosus (ADLE) and 9 chronic discoid lupus (CDL), was conducted to determine the incidence of disorders of conduction (DC), and to study their prognosis and discuss their pathogenicity. The mean age of the group was 38 +/- 16 years, and the mean follow-up period after discovery of the DC was 53 months. Cardiac lesions were present in 49.5 p. cent of the 103 patients with ADLE : pericarditis in 27 p. cent, murmur from lupus endocarditis or cardiomyopathy in 23 p. cent, heart failure in 4.8 p. cent, and hypertension in 17 p. cent. Disorders of conduction were present in 18 (17.5 p. cent) of the 112 patients studied. These included 5 partial right bundle-branch blocks (no complete right bundle-branch block), 2 complete and 3 partial left bundle-branch blocks, 5 complete, 2 first degree, and 1 second degree atrioventricular blocks (AVB). The atrioventricular blocks were usually located in the truncal or fascicular regions, but in 2 cases they were nodal in origin. The 5 complete AVB were associated with ADLE in two cases and CDL in the three other cases. The AVB in the ADLE cases appeared 9 to 20 years after the onset of the lupus, these two patients developing pericardiomyocarditis unaccompanied by disorders of conduction. The three complete AVB occurring during CDL were detected 9 to 18 months after the diagnosis. A fatal outcome was noted in 13 (12.5 p. cent) of the ADLE patients and one of the 9 cases of CDL. Ten-year survival curves showed no difference in prognosis for the groups with or without disorders of conduction, but mortality increased in patients with DC after 10 years. As disorders of conduction were more frequently observed in patients with lupus than in a control population, they can be attributed to either a lupus myocarditis or prolonged administration of synthetic antimalarial agents. Disorders of conduction, and particularly complete AVB are, in fact, observed in patients without pericardiomyocardial lesions, and when they exist usually develop a long time after the onset of the cardiac lesion. All patients had been treated with antimalarials, however, and the onset of the DC was associated with a chloroquine myopathy in some of them. Three of the five complete AVB were observed during the course of CDL in patients without cardiac lesions, this being a supplementary argument for implicating synthetic antimalarials.