To investigate the previously postulated association of systemic lupus erythematosus (SLE) and porphyria 38 patients with various types of porphyria were investigated for clinical and laboratory evidence of a connective tissue disease. Antinuclear antibodies (ANAs) were found in 8/15 (53%) patients with acute intermittent porphyria. These patients were more likely to have had a recent acute attack of porphyria, but only one patient had clinical evidence of SLE. Antinuclear antibodies were not found in any patients with latent acute intermittent porphyria or appreciably in any of the other types of porphyria studied. This finding of ANAs in patients with acute intermittent porphyria may explain the previously described association with SLE. Strict diagnostic criteria need to be used in any one patient as these two disorders have many similar clinical manifestations.