To assess the maternal haematological effects of betamethasone administered in late pregnancy, an automated full blood count was performed before and daily for 5 days after betamethasone in 25 women with a singleton pregnancy between 23 and 33 weeks' gestation. From a mean (+/- SD) baseline level of 11.0+/-2.2 x 10(9)/L, the total white cell count increased significantly to 13.2+/-2.9 x 10(9)/L and 13.5+/-3.1 x 10(9)/L on the first and second day after treatment respectively, returning to baseline on day 3 (p<0.0001, ANOVA). These changes represented a mean increase in the neutrophil count of 35% and a mean decrease in the lymphocyte count of 23%. While there was considerable intersubject variation in the extent of the changes, this study has quantified the leucocytosis induced by betamethasone in late pregnancy, information that may assist with the clinical evaluation of a woman at risk of preterm delivery.