Urinary total, isomer I and isomer III coproporphyrin excretion was determined in 11 patients with Rotor's syndrome, 23 phenotypically normal family members, 16 patients with the Dubin-Johnson syndrome and 20 normal control subjects. Control subjects excreted 24.8 +/- 1.3 per cent (mean SEM) of urinary coproporphyrin as isomer I. Patients with the Dubin-Johnson syndrome excreted 88.9 +/- 1.3 per cent as urinary coproporphyrin I, and patients with Rotor's syndrome excreted 64.8 +/- 2.5 per cent as urinary coproporphyrin I, significantly different from the control subjects and the patients with the Dubin-Johnson syndrome (p less than 0.001). Eight phenotypically normal parents and children of patients with Rotor's syndrome excreted 42.9 +/- 5.4 per cent as urinary coproporphyrin I, intermediate between results in patients with Rotor's syndrome and control subjects (p less than 0.001). Total urinary coproporphyrin excretion was markedly increased in patients with Rotor's syndrome (332 +/- 86 mug/g creatinine) as compared to that in control subjects (p less than 0.001) or obligate heterozygotes (p less than 0.025). With respect to urinary coproporphyrin excretion, Rotor's syndrome and Dubin-Johnson syndrome are both inherited as autosomal recessive traits and are separate pathophysiologic entities. Study of rare but distinct inheritable disorders, such as these, provide insight into the functional dissociation of hepatic transport mechanisms.