The prevalence of chloroquine-resistant Plasmodium falciparum malaria has been increasing in sub-Saharan Africa and parts of South America over the last 2 decades, and has been associated with increased anaemia-associated morbidity and higher mortality rates. Prospectively collected clinical and parasitological data from a multicentre study of 788 children aged 6-59 months with uncomplicated P. falciparum malaria were analysed in order to identify risk factors for chloroquine treatment failure and to assess its impact on anaemia after therapy. The proportion of chloroquine treatment failures (combined early and late treatment failures) was higher in the central-eastern African countries (Tanzania, 53%; Uganda, 80%; Zambia, 57%) and Ecuador (54%) than in Ghana (36%). Using logistic regression, predictors of early treatment failure included younger age, higher baseline temperature, and greater levels of parasitaemia. We conclude that younger age, higher initial temperature, and higher baseline parasitaemia predict early treatment failure and a higher probability of worsening anaemia between admission and days 7 or 14 post-treatment.