Mean platelet volume, platelet count and platelet distribution width were determined in eight male subjects at monthly intervals over a 4-6 month period. Measurements were made on a Coulter S Plus VTM exactly 15 min and 2 h after blood collection. Long-term intra-individual variability for mean platelet volume showed a mean coefficient of variation (CV) of 1.60% at 15 min and 2.01% at 2 h after blood collection; for platelet count a mean CV of 3.18% at 15 min and 2.40% at 2 h; and for platelet distribution width a mean CV of 7.3% at 15 min and 6.5% at 2 h. Platelet parameters are stable over time in the normal male individual.