It has been suggested that the clinical expression of autosomal dominant polycystic kidney disease (ADPKD) is uniform among individuals of a given family. To test this hypothesis, intrafamilial variations in ages at onset of first symptoms, types of first symptoms, serum creatinine concentrations, and renal sizes were evaluated in 131 patients with ADPKD from 36 unrelated families. These parameters were compared in younger and older affected relatives in the same family at a single time, due to difficulties of following them longitudinally. Because the natural course of the disease is to progress with age, it was presumed that disease progression in a given family was nonuniform if older individuals had lower serum creatinine concentrations, and/or smaller kidneys than their affected younger relatives, or if relatives of similar ages had different serum creatinine concentrations and/or kidney sizes. Nonuniform progression was suggested in 38% of affected relatives by serum creatinine concentrations and in 53% by kidney sizes. Ages at onset of first symptoms and types of first symptoms were also different in patients from the same families. These data indicate that phenotypic expression of ADPKD may differ considerably among patients who belong to the same families.