Parasitological data of various malarial studies performed in the Congo where Plasmodium falciparum malaria is holo-endemic in rural and suburban zones, between 1988 and 1991, were analyzed with the intention of establishing diagnosis and prognosis value of Plasmodium falciparum parasitaemia in areas with high perennial transmission. In such an area congolese school-children (6-10 years old) had 88% P. falciparum index, this is the same percentage as that for children hospitalized with a pernicious attack. However, the parasite load is distributed differently; parasitaemia is greater than 6,000 asexual form of P. falciparum/microliters (afPf/microL) in only 4.6% of cases in the former group versus 67% in the second group. A threshold of 10,000 afPf/microliters, above which the Plasmodium infection triggers a febrile attack in semi-immune children, is confirmed in school children in a rural context where the factor of taking antimalarial drugs within the preceding days is negligible; three out of four children with levels above this threshold are febrile versus 4.1% (7 out of 170) with lower blood parasite levels. Some adults were also asymptomatic carriers but much less frequently and with lower mean parasitaemia levels. The parasite load mirrors the clinical severity although this concept can be misleading as an individual prognostic criterion and for hospital studies carried out in areas where multiple drug administration before hospitalisation is common. For the studies recently performed in Brazzaville, the 5% threshold level of parasitized red cells, the WHO severity criterion, was never reached in asymptomatic subject or in cases of simple attack; it was reached in one out of two cases of pernicious attack.