Previous studies have shown depressed serum corrected calcium and phosphate concentrations in acute falciparum malaria. To characterize malaria-associated disturbances in mineral homoeostasis further, serum ionized calcium and intracellular phosphate were measured in 18 patients (10 with falciparum malaria, 8 with vivax malaria) and 10 healthy controls. Six patients (4 falciparum, 2 vivax) had admission serum ionized calcium concentrations below the absolute control range (< 1.15 mmol/litre) and a further six (3 falciparum, 3 vivax) developed ionized hypocalcaemia during treatment. The patients with falciparum malaria had the lowest values at presentation (median [95% confidence intervals in brackets]: 1.17 [1.12-1.23] vs. 1.20 [1.18-1.24] mmol/litre in controls, P = 0.035) in the presence of depressed simultaneous serum parathormone concentrations (1.2 [0.6-1.9] vs. 1.6 [1.1-2.6] pmol/litre; P = 0.05). Admission serum phosphate concentrations were lower in the malaria patients (P = 0.007 vs. controls), especially in those with falciparum malaria (0.85 [0.7-1.1] vs. 1.2 [1.1-1.3] mmol/litre in controls; P = 0.002); patients with falciparum malaria also had significantly lower intracellular phosphate than controls (0.74 [0.58-0.90] vs. 0.88 [0.66-1.04] mmol/litre red cells; P = 0.047). There was a weak association between serum corrected and ionized calcium in the malaria patients (rs = 0.31, n = 18, P > 0.1), but serum and intracellular phosphate correlated significantly (rs = 0.71, n = 17, P < 0.001) with a regression line slope of 0.49 and intercept of 0.27 mmol/litre of red cells.(ABSTRACT TRUNCATED AT 250 WORDS)