Although fever is the characteristic sign of clinical malaria, many Plasmodium falciparum malaria cases in endemic areas do not present with measurable temperature elevations. In a field study in Tanzania, malaria morbidity was defined to be any current self- or parentally reported illness associated with malaria parasite densities higher than those in healthy individuals. Without diagnosis of individual episodes, prevalences of malaria-attributable morbidity of 9.8% in infants, 1.3% in children 1-4 years of age, and 0.6% in those 5-9 years of age were estimated. No illness was considered to be due to malaria in older individuals. In infants, 66.5% of malaria-attributable morbidity episodes corresponded to axillary temperatures < 37.5 degrees C. In older children, most of the episodes due to malaria corresponded to increased temperatures. This age dependence should be considered when designing diagnostic procedures and outcome measures for epidemiologic studies of malaria.