AIM/BACKGROUND: Previous studies have suggested that healthy subjects of African ancestry have lower total white cell counts, neutrophil counts and platelet counts than Caucasian subjects and that, at least among Caucasians, women have higher neutrophil and platelet counts than men. The primary aim of this study was to confirm and quantify the ethnic differences, confirm the sex difference in Caucasians and determine whether there was a similar sex difference in non-Caucasians. A secondary aim was to establish reference ranges for white cell and platelet counts for the different ethnic and sex groups. METHODS: The study population comprised 417 healthy volunteers (201 women and 216 men), of whom 200 were Caucasian, 102 were Afrocaribbean and 115 were African. Full blood counts, including a differential white cell count, were measured using a H.2 automated differential counter. White cell and platelet counts were compared between the three different ethnic groups and between men and women. Reference ranges were determined for each ethnic and sex group. RESULTS: Africans and Afrocaribbeans had lower total white cell, neutrophil and platelet counts than Caucasians and counts were lower in Africans than in Afrocaribbeans. Women had higher neutrophil and platelet counts than men in all ethnic groups. CONCLUSIONS: Sex and ethnic origin should be taken into consideration when assessing white cell and platelet counts.