Using a quantitative sensitive HPLC fluorometric assay for the measurement of plasma and urine porphyrins, we have calculated the renal clearance of uro- and coproporphyrin in normal volunteers and in patients with congenital erythropoietic porphyria and hereditary coproporphyria (HCP). In patients with porphyria cutanea tarda (PCT), only uroporphyrin clearance was calculated. Plasma porphyrin concentrations were measured in patients with chronic renal failure (CRF). All our CRF patients on haemodialysis (10) had raised plasma uroporphyrin, 4 had raised plasma heptacarboxyporphyrins and 2 had raised plasma coproporphyrin, unchanged by haemodialysis. CRF patients with raised uroporphyrins and 7-carboxyporphyrins could be distinguished from PCT in patients with normal renal function, by their higher uro/7-carboxyporphyrin ratio (8.0 vs. 1.7). Renal clearance of coproporphyrin was much greater in HCP than in any of the other groups studied. Since coproporphyrin clearance was less than creatinine clearance in most normal subjects, and because 2 patients with CRF had raised plasma coproporphyrin concentrations, we argue that the kidney is probably not a major source of coproporphyrin, as previously reported. Furthermore, the variability and degree of abnormality in plasma porphyrins in CRF would suggest caution in diagnosing chronic porphyria in patients with renal impairment.