Plasma fibrinogen levels and euglobulin lysis time (ELT) were determined in 84 women with haemoglobin genotype AA (HbAA) and in 38 with haemoglobin genotype AS (HbAS), aged 17-35 years, who were on oral contraceptive pills (OCP). The control group included 23 HbAA and 27 HbAS age-matched, apparently healthy women, who had regular menstruation and no history of OCP usage. The controls showed statistically significant elevations in fibrinogen levels in women with genotype HbAS (+13%; P < 0.05) compared with women with genotype HbAA. Among OCP users, the difference in fibrinogen levels (+5%) between HbAS and HbAA women was not statistically significant. The elevation in fibrinogen levels which was restricted to the HbAA women, was probably caused by OCP use, and may be dependent on the Hb genotype. In contrast, the observed elevations in euglobulin lysis time among OCP users (P < 0.005) were independent of Hb genotype. Thus, while OCP may constitute a risk factor for the development of thromboembolism in women, the S-gene may confer partial protection against this development in women who have the HbAS genotype.