One hundred one adult patients with acute uncomplicated falciparum malaria were treated with pyronaridine. All patients were admitted to the Bangkok Hospital for Tropical Diseases for 28 days to exclude reinfection. Sixty-nine patients (Group I) received pyronaridine 1,200 mg over a three-day period and 32 patients (Group II) received 1,800 mg pyronaridine over a five-day period. Cure rates for the two groups were 63% (38 of 60) for Group I and 88% (23 of 26) for Group II (P<0.05). No RII or RIII type response was seen. Mean fever and parasite clearance times were not significantly different in the two groups. The drug was well-tolerated. In vitro drug sensitivity tests of the paired parasite isolates obtained prior to treatment and after recrudescence indicated that the Plasmodium falciparum isolates of the successfully treated patients had a lower mean concentration for 50% inhibition of growth (IC50) and a much narrower range of the individual IC50 values (15.69 +or- 3.82 ng per ml (mean +or- SD)) as compared with those from the recrudescence cases (22.98 +or- 12.05 ng per ml). Nevertheless, there was no evidence of an increase of the IC50 and IC95 values after recrudescence. The results of the study show that pyronaridine alone at a total dose of 1,800 mg given over five days is well-tolerated in patients suffering from acute uncomplicated malaria and has evident activity against multidrug-resistant falciparum malaria. However, it cannot be recommended for use in Thailand as long as the recrudescence rate is as high as 12%. Further studies of its combinations with other antimalarial drugs are needed.