OBJECTIVE: The objectives of this study were to obtain information on MIS levels in normal and RDS neonates and to investigate the relationship between the RDS prevalence and MIS level in preterm and term neonates. METHODS: Total 131 male neonates were selected randomly and they were consisted of 50 term normal neonates, 15 term neonates with RDS, 50 prematurely born normal neonates, and 16 prematurely born neonates with RDS. Total 131 female neonates were also selected like male neonates. The venous blood was collected from all subjects and measured the level of MIS using ELISA. The ANCOVA was conducted to evaluate any influence of adjusted value of gestational age and body weight on MIS level between normal neonates and neonates with RDS. RESULTS: 1) The MIS levels of female neonates were significantly lower than those of male neonates with no overlap. 2) The MIS levels of normal female neonates were not significantly different from those of female neonates with RDS. 3) There were significant negative relationships between MIS concentration and gestational age (r=-0.777, p<0.001), and birth weight(r=-0.728, p<0.001) in normal rnale neonates. 4) There were significant negative relationships between MIS concentration and gestational age (r=-0.726, p<0.001), and birth weight(r=-0.725, p<0.001) in male neonates with RDS. 5) After adjusting the value of gestational age, the MIS level of male neonates with RDS was significantly higher than that of normal male neonates(p<0.001). 6) After adjusting the value of body weight, the MIS level of male neonates with RDS was significantly higher than that of normal male neonates(p<0.001). CONCLUSION: Male neonates with RDS had higher MIS levels than normal male neonates of the same body weight or same calculated gestational age. The results of this study suggest that MIS may play a causative or important ancillary role in the sexual dimorphism that characterizes the neonatal RDS and may be used as a predictive marker of RDS in male neonates.