A population of 183 Zairean infants and young children (0.5 to 48 months old) who lived in rural Bas-Zaire has been examined in regard to hemoglobin (Hb), hematocrit (Hct) and mean corpuscular hemoglobin concentration (MCHC). The mean Hb was 9.03 g/dL, Hct 28.4%, and MCHC 30.7 micrograms/dL. These means are below normal by WHO and/or American Academy of Pediatrics criteria. Although there was no significant sex difference in the mean hematoglobin measurements, boys more than 24 months old had a higher mean Hb (9.98 g/dL) than girls (8.59d g/L) of the same age. Approximately 80%, 76% and 56% of children had Hb < 11 g/dL, Hct < 34% and MCHC < 32 micrograms/dL respectively. The highest prevalence of anemia was between the age of 12 and 18 months. Severe anemia (Hb < 8 g/dL) was observed in 30.65% of the study population (26.6% boys and 34.8% girls). Approximately one third of children had very low Hct levels (< or = 25%). Based on MCHC, iron deficiency was likely responsible of the anemia in 61% of the children. Both Hb (p < 0.05) and Hct positively correlated with the mother's level of education. Because of the many detrimental effects of iron deficiency in infants and young children on immunity, psychomotor, behaviour and mental development routine assessment of iron status and correction of iron deficiency should become a part of the Surveillance Programme in this population.