Smoking in hospitals is now forbidden. In several hospitals, however, women in labour are allowed to smoke in designated smoking areas. This study assesses whether smoking during labour increases the carboxyhaemoglobin concentrations in maternal and cord blood, taking into account the number of cigarettes smoked during pregnancy, duration of labour and parity. Women were questioned on their smoking behaviour shortly after delivery. A total of 295 mother-infant pairs were included in the study. Of the 94 (31.9%) smokers, 33 smoked during labour at home only and 34 during labour at the hospital. For newborns of smokers, the daily ration of cigarettes smoked by the mother during pregnancy and the number smoked during labour explained respectively 10.4% and 10.8% of the residual variance of carboxyhaemoglobin in cord blood. Smoking during labour significantly increases carboxyhaemoglobin levels. It should be prohibited in hospitals.