Chloroquine has been the standard treatment for Plasmodium vivax malaria for more than 40 years in most regions of the world. Recently, however, chloroquine-resistant P. vivax has been reported from Oceania, several parts of Asia, and South America. In order to assess the situation in Thailand, 886 patients with vivax malaria who were admitted to the Bangkok Hospital for Tropical Diseases from 1992 to 1997 were followed prospectively. Most of the patients had been infected on the western border of Thailand and were experiencing their first malarial infection when admitted. All received oral chloroquine (approximately 25 mg base/kg body weight, administered over 3 days) and then were randomized to receive primaquine (15 mg daily for 14 days) or no further treatment. All the patients were initially responsive to chloroquine, clearing their parasitaemias within 7 days, and there were no significant differences in the clinical or parasitological responses between those treated with primaquine and those given no further treatment. Plasmodium vivax parasitaemias re-appeared within 28 days of chloroquine treatment in just four patients. In each of these four cases, re-treatment with the same regimen of chloroquine resulted in eradication of the parasitaemia, with no further appearance of parasitaemia during the next, 28-day, follow-up period. These data indicate that virtually all acute (i.e. blood-stage) P. vivax infections acquired in Thailand can still be successfully treated with chloroquine