A malaria prevalence survey was carried out in young children in northern Ghana between October 1990 and September 1991, in an area with continuous mortality and morbidity surveillance. There was marked seasonal variation in malaria deaths, reported fevers, parasite rates and mean parasite densities, with parasite rates reaching 85-94% in the wet season. The monthly numbers of malaria deaths were highly correlated with rainfall in the previous 2 months (r = 0.90, P < 0.001). Parasite rates were highest in the oldest children (5-7 years), but parasite densities and rates of febrile illness were highest in those 6-11 months old. Haemoglobin levels were also at their lowest in this age group. The predominant species, Plasmodium falciparum, was present in 71% of all blood films. Febrile illness was well recognized by mothers, but it was not possible to construct a simple clinical diagnostic algorithm which would identify even 50% of children with high levels of malaria parasitaemia (> or = 4000 parasites/microL). Malariometric indicators appear to have changed little in this area since a previous survey in 1955.