Nitric oxide is an important host defence molecule as well as being a mediator in many pathophysiological processes. To investigate its role in severe malaria, we measured plasma nitrate and nitrite concentrations in 70 children with malaria (54 with severe malaria) and 48 control subjects (33 with medical conditions and 15 surgical patients). We related these measurements to plasma lactate concentrations, an established marker of disease severity in malaria. Plasma lactate levels were significantly elevated in patients with deep coma (P = 0.0007) and those with a fatal outcome, but mean nitrogen oxide concentrations were not significantly different in the 2 outcome categories and were not related to depth of coma (P > 0.5). In patients whose cerebrospinal fluid (CSF) was examined, lactate concentrations were elevated in fatal cases (geometric mean 8.2 mmol/L, n = 5) compared with survivors (3.4 mmol/L, n = 13; P = 0.032); corresponding CSF nitrogen oxide concentrations were 10.7 microM in fatal cases compared with 12.5 microM in survivors (P = 0.5). Plasma nitrogen oxide concentrations were negatively correlated with admission parasitaemia (r = -0.41, n = 70; P < 0.0001). In our population, elevations of plasma lactate, but not nitrite or nitrate, reflected disease severity in malaria.