During an 18-month period, 10 consecutive Mexican mestizos with a dinical marker associated with a primary hypercoagulable state were studied. The assessment of the sticky platelet syndrome (SPS) was done by the method described by Mammen. In addition, the activated protein C resistance phenotype, coagulation protein C activity and antigen, coagulation protein S, antithrombin III, plasminogen, tissue-type plasminogen activator activity, plasminogen activator inhibitor activity, plasminogen activator inhibitor type 1, IgG and IgM isotypes of anti-phospholipid antibodies, homocysteine levels, the factor V gene Leiden mutation, the 677 C->T mutation in the 5,10-methylen-tetrahydrofolate-reductase (MTHFR), and the G20210A polymorphism in the 3'-untranslated region of the prothrombin gene were studied. Six patients with the SPS were identified: only one displayed this as the single thrombophilic abnormality; in five others, additional thrombosis-prone conditions were found: heterozygosity for the MTHFR 677 gene mutation in five cases; and, in one case each, heterozygosity for the factor V Leiden mutation, heterozygosity for the factor II G20210A mutation, and antiphospholipid antibodies in another. Four of the six patients had a family history of thrombophilia. All patients were treated with aspirin and no new vasoocclusive episodes have been recorded. SPS in not an infrequent finding in Mexican mestizo thrombophilic patients and may contribute to thrombophilia.