We have studied the effects of oral contraceptive (OC) use and of subsequent multivitamin supplementation on several hematological parameters. Hemoglobin (Hb), hematocrit (Ht), mean corpuscular volume (MCV), mean corpuscular hemoglobin concentration (MCHC), mean corpuscular hemoglobin (MCH), and serum iron status (serum iron, total iron binding capacity and ferritin) were tested in groups of women with and without OC in view of preconceptional status. The group taking sub-50 OC comprised 28 women while 31 women were included in the group of non-OC users. Blood samples were taken on days 3 and 23 of the first cycle to obtain baseline values of each analyte. Multivitamin supplementation started on day 1 of the second cycle and this was continued daily throughout three consecutive cycles until the end of the study. Comparison of the baseline values of each parameter as determined on days 3 and 23 of the first cycle of the two groups revealed no significant different hematological parameters due to OC-use unlike the parameters of serum iron status which were all significantly increased for the group of OC-users as compared to the group of non-OC users. The effects of multivitamin supplementation on the hematological and iron status parameters were studied within each group. The consistency of each effect of multivitamin supplementation between the two groups was also tested. A multivitamin supplementation significantly decreased MCHC within either group, and caused increases of MCV, whereas Ht and MCH remained unaltered. With respect to the iron balance, serum iron and total iron binding capacity significantly increased, whereas serum ferritin decreased during multivitamin supplementation.(ABSTRACT TRUNCATED AT 250 WORDS)