OBJECTIVE: We sought to examine, in a large cohort of infants within a definable population of live births, the incidence, risk factors, treatments, complications, and outcomes of meconium aspiration syndrome (MAS). DESIGN: Data were gathered on all of the infants in Australia and New Zealand who were intubated and mechanically ventilated with a primary diagnosis of MAS (MASINT) between 1995 and 2002, inclusive. Information on all of the live births during the same time period was obtained from perinatal data registries. RESULTS: MASINT occurred in 1061 of 2,490,862 live births (0.43 of 1000), with a decrease in incidence from 1995 to 2002. A higher risk of MASINT was noted at advanced gestation, with 34% of cases born beyond 40 weeks, compared with 16% of infants without MAS. Fetal distress requiring obstetric intervention was noted in 51% of cases, and 42% were delivered by cesarean section. There was a striking association between low 5-minute Apgar score and MASINT. In addition, risk of MASINT was higher where maternal ethnicity was Pacific Islander or indigenous Australian and was also increased after planned home birth. Uptake of exogenous surfactant, high-frequency ventilation, and inhaled nitric oxide increased considerably during the study period, with >50% of infants receiving > or =1 of these therapies by 2002. Risk of air leak was 9.6% overall, with an apparent reduction to 5.3% in 2001-2002. The duration of intubation remained constant throughout the study period (median: 3 days), whereas duration of oxygen therapy and length of hospital stay increased. Death related to MAS occurred in 24 infants (2.5% of the MASINT cohort; 0.96 per 100,000 live births). CONCLUSIONS: The incidence of MASINT in the developed world is low and seems to be decreasing. Risk of MASINT is significantly greater in the presence of fetal distress and low Apgar score, as well as Pacific Islander and indigenous Australian ethnicity. The increased use of innovative respiratory supports has not altered the duration of mechanical ventilation.