Assessment of the status of some micronutrients is complicated by the acute phase response to infection. We investigated whether 25-hydroxyvitamin D (25OHD) is an acute phase reactant by measuring the effect of a malarial infection on plasma 25OHD level. Blood samples were taken from patients with Plasmodium falciparum malaria daily during the course of their stay in hospital and at a follow-up session 2-6 weeks after discharge. 25OHD was measured by radioimmunoassay. Time course data for the 14 subjects who provided samples for at least the first 2 d in hospital showed no change in 25OHD level during the acute infection. For the 14 subjects with follow-up results, there was no difference between median 25OHD level at admission (25.6 ng/ml, interquartile range (IQR) 23.0, 40.2) and at follow-up (25.2 ng/ml, IQR 19.2, 32.1; P=0.084). 25OHD level appears to be unaffected during the course of a severe malarial infection and thus can be used as a measure of vitamin D status even in subjects who are currently ill.