Urinary coproporphyrin isomers were measured in 17 patients with Rotor's syndrome, 65 phenotypically normal relatives of 14 patients from eight families, and 21 normal subjects. Coproporphyrin I was elevated in Rotor's syndrome (p < 0.001) and, to a lesser degree, in phenotypically normal parents (p < 0.005), children, and siblings (p < 0.025) as compared to normal controls. Coproporphyrin III in patients, parents, and children did not differ from that in normal controls. Genetic analysis was consistent with transmission of Rotor's syndrome as an autosomal recessive trait with respect to urinary excretion of coproporphyrin I. After i.v. injection of σ-aminolevulinic acid, urinary coproporphyrin I increased to a similar extent in Rotor's syndrome and in normal subjects; urinary coproporphyrin III excretion was less in Rotor's syndrome than in normal subjects.