BILATERAL congenital polycystic kidney disease has been justly called "the most important malformation of the urinary tract." A review of the literature of the last 20 years, however, reveals that approximately 75% of the cases are not detected or recognized clinically, and an accurate diagnosis is not established until postmortem examination. The following series of 94 cases have been observed in the Cleveland Clinic from January, 1935, to January, 1951. The diagnoses have been based on detailed histories in which particular attention has been focused on family histories, complete physical examinations, laboratory data, and roentgenologic visualization of the urinary tracts. The latter were secured by intravenous urography, retrograde pyelography, or a combination of the two procedures. Additional diagnostic evidence has been added in some instances either upon surgical exploration or at the time of autopsy examination. Most of the cases have been followed by recheck urologic survey or by correspondence