Malaria epidemics in highland areas of East Africa have occurred with increasing frequency since the late 1980s, but the actual risk of Plasmodium falciparum infection in children and adults during these epidemics has not been well characterized. During a malaria epidemic in a highland area of Kenya, risk of infection was assessed in 50 adults (> or =18 years old) and 32 children (< or =8 years old) after treatment and parasitologic clearance with sulfadoxine-pyrimethamine treatment. Over a 10-week period, 36 of the 82 study participants (43.9%) became infected. The risk of infection was similar in children and adults (hazard ratio for children = 1.21, 95% CI: 0.63-2.33). These findings contrast with the age-related protection from infection reported in areas of stable, intense transmission, and demonstrate that during malaria epidemics, both children and adults in highland areas of Kenya are at high risk of infection.