PER S I S T E N C E of the primitive cloaca with absence of the urethra, absence of the external genitalia, and anal atresia is described in one sibling of a monoamniotic-mon ochorionic twin birth. The infant also had severe bilateral renal dysplasia but no evidence of pulmonary hypoplasia. CASE REPORT Mrs. B. is a 23-year-old secundigravida. Her first pregnancy was spontaneously aborted after two months. The present pregnancy, complicated by pre-eclampsia, was treated with hospitalization, bed rest, and pressure pills during the final two months. A twin pregnancy was diagnosed at this time. Desiccated thyroid, taken for a period of ten years because of a history of being overweight, was discontinued when the pregnancy was first diagnosed. The mother had been adopted and her family history was unavailable. Paternal family history was positive for twin births, but negative for renal disease, fetal wastage, or early death in the neonatal period. A subsequent third pregnancy has resulted in a normal girl. Spontaneous rupture of the fetal membranes occurred four hours prior to delivery. Twin A, the subject of this report, weighed 2,835 gin, and was delivered by elective low forceps. Apgar scores were 8 at one minute and 9 at five minutes. The delivery of Twin B was complicated by prolapse of the umbilical cord. At birth his weight was 3,246 gm and physical examination was that of a normal boy. The placenta was monoamniotic; both infants were blood type A Rh-negative. Twin A was transported to the Newborn Intensive Care Unit at the University of New Mexico School of Medicine. On admission the infant appeared alert and in no distress. Gestational assessment was consistent with a 40-week, appropriate-for-gestational age infant. The length was 48 cm and head circumference was 34 cm; the heart rate was 150/minute and respiratory rate 56/minute. Abnormal physical findings were limited to the abdomen and perineat area (Fig. 1). The abdomen was distended and had decreased muscle tone. A left upper quandrant mass, right flank mass, and suprapubic mass were palpated. Only two umbilical vessels could be identified. There was a rudimentary scrotal sac without palpable gonads in the sac or inguinal area. The remainder of the perineum was smooth, with no anal orifice or dimple.