The impacts of acute falciparum malaria on body weight and the host and parasite factors predictive of change in body weight were characterized in 465 prospectively studied children in an endemic area of southwest Nigeria. Pre-treatment weights were significantly lower than the 14 to 28-day post-treatment weights (P = 0.0001). In 187 children, fractional fall in body weight (FFBW) exceeded 4.9%. FFBW correlated negatively with age and body weight (P = 0.014 and 0.0001, respectively), but not with enrollment parasitaemia. In a multiple regression model, an age < or =5 years (AOR = 2.03, 95% CI 1.2-3.2, P = 0.003), a hematocrit < or =29% (AOR = 1.6, 95% CI 1.0-2.3, P = 0.037), and a body weight < or =9.6 kg (AOR = 5.4, 95% CI 1.7-20, P = 0.003) were independent predictors of FFBW > or =5% at presentation. Children who, after initial clearance, had recurrence of their parasitaemia within 28 days had a significantly higher propensity not to gain weight than children who were aparasitaemic after treatment (log-rank statistic 6.76, df = 1, P = 0.009). These results indicate that acute malaria contribute to sub-optimal growth in young children and may have implications for malaria control efforts in sub-Saharan Africa.