Levels of procalcitonin (ProCT) have been found to be elevated in individuals with severe bacterial infections such as sepsis and peritonitis, and this correlates well with the severity of the disease. Recently, increased levels have been described in melioidosis and Plasmodium falciparum malaria. In this study ProCT levels were measured in 27 Thai patients with complicated malaria before and during/after treatment with artesunate and mefloquine. Initial parasite counts averaged 290,680/microl (range = 533-1,147,040). On admission, ProCT levels were elevated in all but one patient (median = 40 ng/ml, range = 0.04-662, normal values < 0.5 ng/ml). With treatment, levels decreased to 1.3 ng/ml (range = 0.01-6.5). Nitrite/nitrate levels in patients were higher than in controls throughout the study. The ProCT levels correlated with initial parasite density (P < 0.05), which is a marker of disease severity, and with nitrite/nitrate levels (P < 0.05). Based on the changes of ProCT levels over the course of the disease a possible role in the acute-phase reaction seems likely.