A total of 225 pre-school Nigerians were studied to evaluate the effect of anaemia on the respiratory rate of febrile children, and the influence of this on the reliability of the WHO criteria for the clinical diagnosis of pneumonia in the primary health care setting. Malaria was the commonest cause of febrile illness (63.1 pc). The prevalence of severe anaemia was 28.0 pc. The mean respiratory rate of anaemic children (53.9 +/- 12.8/min) was significantly higher than that of the non-anaemic (48.4 +/- 12.7/min; p = 0.011). Mean body temperature was not significantly different in both groups. Haematocrit was negatively correlated with respiratory rate. The positive correlation observed between temperature and respiratory rate was enhanced by decreasing haematocrit. More of the severely anaemic children (68.6 pc) exceeded the WHO respiratory rate threshold for diagnosis of pneumonia than the moderately anaemic (55.4 pc) or non-anaemic (36.1 pc). The specificity of the WHO criteria for clinical diagnosis of pneumonia decreased with decreasing haematocrit.