Recently, the asymmetric distribution of phospholipids in eukaryotic cell membranes has been appreciated and been found to be dependent on the activity of a number of enzymes. The expression of phosphatidylserine (PS), a negatively charged phospholipid, on the platelets of patients with polycythemia vera (P vera) and essential thrombocythemia (ET) was compared to that in normal individuals. The effect of platelet aggregation on PS expression was determined. Exposure of PS on platelets obtained from patients with P vera and ET and from age- and sex-matched healthy volunteers was measured by fluorescein-labeled Annexin V binding to platelets and by the platelets' thrombin-generating capacity determined by the prothrombinase assay. PLatelet prothrombinase activity (mean +/- standard deviation [SD]), as measured by thrombin generation, was 2.32+/-2.2 micro/mL in the P vera group and 1.55+/-1.0 micro/mL in the control group (p=0.3). PS expression as measured by Annexin V binding (mean +/- SD) was 2.6+/-2.4 % in the P vera group versus 1.55+/-1.2% among controls (p=0.03). In the ET group, prothrombinase activity (mean +/- SD) was 1.0+/-0.6 micro/mL and 2.1+/-0.9 micro/mL in the control group (p=0.006). Annexin V binding (mean +/- SD) was 4.8+/-4.2% in the ET group and 2.77+/-2.1% among control subjects (p=0.09). When the prothrombinase assay was performed after addition of adenosine diphosphate (ADP) to the platelets, there was a significant increase in thrombin generation in the myeloproliferative disorder (MPD) group (3.1+/-2.0 micro/mL) compared to the thrombin generated by unstimulated myeloproliferative disorder platelets (2.07+/-1.69 micro/mL) (p=0.0006). An increase in thrombin generation was seen in the ADP-stimulated platelet samples in all ten paired samples studied. Likewise, the addition of ADP to control platelets increased thrombin generation from 2.0+/-1.0 micro/mL in unstimulated platelets to 4.3+/-1.6 micro/mL in ADP-treated platelets (p=0.0006). Thrombin generation increased in all of the ADP-stimulated platelet samples compared to the untreated platelets. There was however, no difference in the increased thrombin generation when ADP-stimulated platelets from MPD patient and control subjects were compared (p=0.3). Results indicate that some patients with MPDs may show increased PS expression on platelet surface. When analyzed overall, there was a tendency toward greater PS expression in the P vera and ET patient groups; however, the increase did not reach statistical significance. This increase was noted in both the prothrombinase assay the Annexin V binding assay. We have also shown that stimulation of platelets by addition of the agonist ADP results in enhanced PS expression, which appears increase the thrombogenic potential of the platelets as demonstrated by the enhanced thrombin generation demonstrated by these platelets in the prothrombinase assay. There was no difference in the degree of PS expression in response to ADP stimulation between MPD and control platelets. Results show that PS expression and platelet-dependent thrombin generation is variable in patients with MPDs. This expression is increased after platelet aggregation occurs. The role of PS expression in the thromboembolic complications of MPD patients should be studied further.