The incidence and outcome of babies suffering from meconium aspiration syndrome (MAS) has been analysed in a 6-year review of 38,395 births at Guy's Hospital and Queen Charlotte's Maternity Hospital. One in 533 births (0.2%) was complicated by MAS, the mortality rate for which was 1 in 12. There was also a number of babies identified as possibly suffering from 'subclinical MAS' but no deaths occurred in this group. Males were more affected than females.