Does cigarette smoking in pregnancy explain the increased risk of adverse perinatal outcomes that occur with maternal asthma or does it compound the effect? Using population based birth records, a retrospective analysis was conducted of all singleton pregnancies in South Australia over 10 years (1999-2008; n=172 305), examining maternal asthma, cigarette smoking and quantity of smoking to estimate odds ratios. Compared with nonasthmatic females who did not smoke during pregnancy, both asthmatic females who smoked and those who did not smoke during pregnancy had a significantly increased risk of gestational diabetes, antepartum haemorrhage, polyhydramnios, premature rupture of membranes, emergency Caesarean section, and the child being small for gestational age and having congenital abnormalities. These associations suggest that asthma, independently of maternal smoking, increases the risk of these adverse perinatal outcomes. Maternal smoking was itself associated with an increased risk of a number of poor neonatal outcomes, with a dose-response relationship observed. Notably, maternal asthma combined with cigarette smoking significantly increased the risk of preterm birth and urinary tract infections to a greater degree than with either exposure alone. Maternal asthma and cigarette smoking during pregnancy are both independently associated with adverse perinatal outcomes and, combined, compound the risk of preterm birth and urinary tract infections.