An increase in parasitaemia is not uncommon after initiation of treatment for Plasmodium falciparum malaria, but its exact significance is unknown. The time-course of parasitaemia was assessed retrospectively in 33 patients with severe imported malaria. In 19 patients (group 1) mean parasitaemia (+/- SEM) fell promptly after starting quinine treatment, from 24.9 +/- 4.1% on day 0 to 9.7 +/- 2.3% on day 1 and 1.8 +/- 0.7% on day 2. In 14 other patients (group 2), parasitaemia did not change significantly or increased, with mean parasitaemia (+/- SEM) of 9.5 +/- 2.1% on day 0, 17.2 +/- 2.6% on day 1, and 3.7 +/- 1.8% on day 2. Simplified acute physiology scores on admission (mean +/- SEM) were 17.4 +/- 1.4 in group 1 and 11.7 +/- 1.0 in group 2 (P = 0.006). The mean number of complications of malaria per patient (+/- SEM) was 2.9 +/- 0.5 in group 1 and 1.6 +/- 0.3 in group 2 (P = 0.046). Two group 1 patients died. Initially, more than 95% of peripheral blood parasites were tiny and small rings in both groups, and this distribution was unchanged on day 1, suggesting that the parasitaemia increase in group 2 was not due to release of sequestered mature parasites. In severe falciparum malaria, a rise in parasitaemia after treatment initiation may be of favourable prognostic significance and should not lead to aggressive therapeutic approaches such as exchange transfusion.