The clinical course and the pathologic features of 29 patients with the prune belly syndrome (PBS) are reviewed. There were 26 males and 3 females. In addition to the classical triad of deficient abdominal musculature, urinary tract abnormalities, and cryptorchidism, a broader spectrum of other defects was found including musculoskeletal (58%) and gastrointestinal (31%) abnormalities. Genital anomalies were present in all three female patients. Many of these defects may be inapparent at birth, but are the cause of morbidity and mortality later in life. Severe urinary tract maldevelopment and pulmonary hypoplasia as part of the oligohydramnios syndrome was the most common cause of perinatal deaths. In these patients, major portions of the renal parenchyma were dysplastic, but in survivors, renal dysplasia, when present, was minor by comparison, and affected less than 1/3 of the parenchyma. Although several questions remain unanswered, we believe that the PBS results from the effect of one or more teratogenic agents on the somatic mesoderm, producing inappropriate mesenchymal development and inadequate mesenchymal-epithelial interactions that lead to abnormal development and dilatation of some of its derivatives (abdominal muscles, ureter, bladder, prostate, urethra, and gubernaculum). Although abnormalities in derivatives of the intermediate mesoderm (kidney) may also be produced by the injurious agent(s), they are more likely a result of urinary obstruction. Abnormalities in other organs and systems are the consequence of oligohydramnios.