OBJECTIVE Many cases of porphyria cutanea tarda have been described in association with human immunodeficiency virus (HIV) infection in young individuals. The link between hepatitis C virus (HCV) and porphyria cutanea tarda is even stronger as more than 50% of patients who have this diagnosis in Italy, France, or Spain are also infected by HCV. To study the role of viral infections on the metabolism of porphyrins, we measured the urinary porphyrin levels in patients with HIV and HCV infections. METHODS Survey; prospective study. METHODS University Hospital of Strasbourg, France. METHODS Sixty-one HIV-positive patients, 56 HCV-positive patients, 60 HIV- and HCV-positive patients, and 51 HIV- and HCV-negative control subjects were randomly selected. None had clinical signs of porphyria or a familial history of porphyria. METHODS The porphyrin-excretion profile was determined by high-performance liquid chromatography on fresh urine samples. The HIV and HCV viremias were quantified in the serum by the branched DNA assay. Measures were planned before data collection began. RESULTS The porphyrin-excretion profile typical of porphyria cutanea tarda was found in only 1 of 177 patients. In the remaining 176 patients, the mean coproporphyrin level was significantly raised in HCV-positive patients and even higher in patients who were HIV- and HCV-positive. The coproporphyrin level was not correlated to the alanine aminotransferase level, the CD4+ cell count, or the HCV and HIV viremias. CONCLUSIONS In cases of infection with HIV, HCV, or both, the development of a porphyria cutanea tarda urinary profile is a rare event (0.56% in this study), but coproporphyrin excretion is increased. This could be related to hepatic changes induced by the viruses. Our results do not support the hypothesis of a direct viral effect on the porphyrin metabolism. Infection with HIV, HCV, or both may be a major triggering factor, but is not sufficient to induce porphyria.