Erythrocyte sedimentation rate (ESR), C-reactive protein (CRP) and routine hematological parameters were reviewed in 258 patients with acute malaria and compared to a control group of 120 patients with other febrile illnesses after visiting malaria endemic areas. Thrombocytopenia was found in 80% of the malaria patients compared to 13% in controls (p less than 0.01). The malaria patients also had lower white blood cell counts and marginally lower hemoglobin values than control patients. No major differences were found in ESR or CRP values. Furthermore, there were no major differences in the hematological parameters between patients infected with different malaria species, or between patients with different ethnic background. Thrombocytopenia (platelet count less than 150 x 10(9)/l) had a predictive value positive of 56% and a predictive value negative of 95% for malaria in a febrile patient coming from an endemic area. Thus, the risk of malaria in a febrile thrombocytopenic patient coming from an endemic area was 56%, while the risk that another patient with a normal platelet count still had malaria was 5%.