OBJECTIVE: Animal studies suggest that severe antenatal spinal cord injury inhibits fetal breathing movements, and may thus delav pulmonary development. Our objective was to determine if development of pulmonan maturity (PM) is affected hy the degree of fetal hydrocephalus secondary to meningomyelocele (MMC), and if FM study results correlate with outcome in such neonates. STUDY DESIGN: The records of all third trimester fetuses with isolated MMC and hydrocephalus delivered from 1989 to 1995 were reviewed. During this period our policy was to obtain mature PM studies in all MMC cases prior to elective cesarean deliver)'. Deliverv without mature PM studies occurred if the patient labored spontaneously or the gestation reached term. Cases were divided into two groups according to whether or not mature PM studies were ever achieved prior to delivery, and outcomes were compared. RESULTS: Complete records were available on 30/36 neonates; If) had mature PM sttidies prior to deliver) and 14 had immature PM studies. Fetuses with immature PM studies had significantly larger head circumferences (HC, 37.4 ±1.1 cm vs 33.9 ±1.0, p = 0.02), were significantly heavier (3616 ±14 gm vs 3279 ±110, p = 0.04), and a greater percentage had a HC > 2 S.D. (71% vs 38%, p = 0.06). Fetuses with and without mature PM studies were not significantly different with regard to gestational age at deliver)-, 5 Apgar score, need for respirator or oxyhood therapy, days of respiratory therapy required, days in hospital, or survival. CONCLUSIONS: Nearly half of all fetuses with MMC and hydrocephalus have immature PM studies prior to delivery; those with macrocephaly are the most likely to be immature. However, failure to achieve mature PM sttidies is unrelated to ultimate neonatal outcome. PM amniocentesis prior to deliver) may be unnecessary.