A report is presented of a male infant with prune belly syndrome (PBS) in whom bladder enlargement was detected by ultrasound (US) as early as 13 weeks of gestation. Subsequent fetal US identified progressive urinary tract dilatation, ascites and oligohydramnios. At 22 weeks, the fetal bladder was drained under US guidance. A gradual resolution of oligohydramnios was detected on US performed after 26 weeks of gestation. Delivery by cesarean section was performed at 34 weeks of gestation. The newborn had typical features of PBS with a vesico-cutaneous fistula but did not show respiratory distress. Imaging studies showed hypoplastic left kidney, slightly dysplastic but functioning right kidney, megaureter, megacystis, vesicoureteral reflux and dilated prostatic urethra. The early detection of genitourinary system abnormalities and serial US suggest that a distal urethral obstruction may be the underlying mechanism of PBS. Spontaneous (or therapeutic) intra-uterine decompression of the bladder might ameliorate obstructive nephropathy and oligohydramnios, allowing adequate lung development. © 1997 Japan Pediatric Society.