Platelet counts (PC) obtained simultaneously from capillary blood (CB) are generally lower than those from venous blood (VB). We quantified this difference in 17 patients with low platelet counts (LPC) and 18 healthy volunteers with normal platelet counts (NPC). The reproducibility of the counts in these 2 groups of subjects was also evaluated. The mean venous platelet count (VPC) and the mean capillary platelet count (CPC) were 67 +/- 30 x 10(3)/ul (+/- SD) and 61 +/- 23 x 10(3)/ul (p = 0.012) in the LPC, and 264 +/- 44 x 10(3)/ul and 234 +/- 45 x 10(3)/ul (p = 0.00016) in the NPC respectively. The mean difference (d) in the PC between VB and CB were 9.4 +/- 13.1 x 10(3)/ul and 19.4 +/- 17.6 x 10(3)/ul in the LPC and NPC respectively. The coefficients of variation (CV) of double counts for VB and CB were 8.1 +/- 8.3% and 9.8 +/- 8.6% for LPC, and 2.3 +/- 1.6% and 2.5 +/- 2.2% for NPC respectively. In conclusion, VPC was frequently (82.9% or 29/35 cases) higher than CPC. In addition, in patients with LPC, the agreement between VPC and CPC was poor and the counts were less reproducible.