BACKGROUND There is increasing evidence suggesting the contribution of platelets in the vaso-occlusive phenomena found in sickle cell anaemia. This study is aimed at using simple, inexpensive parameters to determine the role of platelets in the steady state and vaso-occlusive crisis state of Nigerian sickle cell anaemia patients. METHODS The circulating platelet aggregate (CPA) ratio, platelet factor-3 availability (PF-3) and platelet counts of 60 adult Nigerian sickle cell anaemia patients were studied. RESULTS The CPA ratio in the sickle cell anaemia (SCA) patients in steady state (SS) was 0.93 +/- 0.05, 0.89 +/- 0.04 during vaso-occlusive crisis (VOC) and 0.98 +/- 0.02 in the control group (C). The values in the vaso-occlusive crisis and in steady state were significantly lower than in the control group (P < 0.05). PF3 availability in steady state and vaso-occlusive crisis were 29.7 +/- 4.0 secs and 28.4 +/- secs respectively. The times are significantly shorter when compared with the control group with a time of 36.2 +/- 4.3 secs (P < 0.05). There was however no significant difference between the two sickle cell groups. Platelet count was significantly raised in the steady state patients 224.3 +/- 46.3 x 10(9)/L when compared with controls of 196.6 +/- 39.3 x 10(9)/L (P < 0.05). There was a significant fall during VOC to 140.6 +/- 36.3 x 10(9)/L (P < 0.05). The difference between the two sickle cell groups is significant (P < 0.05). CONCLUSIONS This study indicates varying degrees of partial activation of platelets in vivo in the steady state and vaso-occlusive crisis state of sickle cell anaemia. It support's a contribution of platelet to the vascular occlusion that underlies much of the morbidity in the disease.