When travellers return from malaria-endemic areas and present to hospital with fever, microscopy of blood smears remains the leading method to verify a suspected diagnosis of malaria. Additional laboratory abnormalities may, however, also be indicative of acute malaria infection. We monitored prospectively a group of patients with imported Plasmodium falciparum (n=28) or P. vivax/P. ovale (n=12) infection, respectively, and assessed haemoglobin, leucocytes, thrombocytes, C-reactive protein, coagulation factor II-VII-X, lactate dehydrogenase and bilirubin during 7 d of admission and weekly until d 28. For comparison, admission values of a group of febrile patients with suspected malaria, but with negative blood slides, were also assessed (n=66). The thrombocyte, leucocyte counts and coagulation factor II-VII-X were significantly lower in the malaria group compared to the non-malaria group, whereas the C-reactive protein, lactate dehydrogenase and bilirubin were significantly higher in the malaria group. The differences were particularly strong with falciparum malaria. By contrast, haemoglobin levels were not affected. In conclusion, our study emphasizes the role of a few commonly analysed laboratory parameters, in particular thrombocyte counts, in guiding the clinician managing a returning traveller with fever.