Sub-Saharan Africa faces increasing levels of resistance of Plasmodium falciparum parasites to the first-line drug pyrimethamine-sulphadoxine (SP). Successful treatment with SP is reported to induce gametocytes and drug resistance may further increase gametocytaemia after treatment. Treatment success, gametocyte prevalence and gametocyte density were determined in 224 asymptomatic children in western Kenya on day 7 after treatment with SP. Treatment failure (R2 or R3 resistance) was observed in 22% of the children. The relative risk to show gametocytes on day 7 after treatment in children with treatment failure was 4.1 (95% CI 1.4-11.6) times higher compared to children with a sensitive infection, after adjustment for age and trophozoite density at the start of treatment. In addition, the gametocyte density was also higher upon SP treatment failure. These findings are reason for concern, as the increased gametocyte prevalence and density after SP treatment failure may increase the spread of SP-resistant strains in the population.