Leukocytosis may develop in women undergoing ovulation induction. The production of blood leukocytes and their numbers in circulation are regulated by complex interactions involving endogenous haematopoietic cytokines, such as granulocyte-colony stimulating factor (G-CSF), monocyte-colony stimulating factor (M-CSF), and interleukins. The purpose of this prospective study was to explore the presence of leukocytosis in women who receive urinary menotrophins, and to determine whether haematopoietic cytokines are changed in the stimulation process. Controls were volunteers of the same age range, not taking any medication, who received daily saline injections. Subjects underwent phlebotomy at defined points for determination of complete blood counts, G-CSF, M-CSF, and interleukin-6 concentrations. Baseline white blood cell (WBC) counts were similar in patients and controls. In menotrophin-treated patients the WBC counts rose significantly (4.19 +/- 0.28 to 6.37 +/- 0.71) during the stimulation and peaked in the luteal phase (P = 0.037). In contrast, WBC counts decreased in controls. Other leukocytic lineages were not affected. In treated patients G-CSF concentrations rose significantly (P = 0.028 versus controls), while changes in M-CSF and interleukin-6 were not significant.