A retrospective study is reported of 160 patients with definite or classical rheumatoid arthritis, in which changes in haemoglobin concentration were analysed against changes in various routine clinical and laboratory indices of disease activity over a period of time. Although statistically significant correlations were found between haemoglobin concentrations and the articular index of joint tenderness, serum albumin concentration, and erythrocyte sedimentation rate at one point in time, significant relationships in changes in haemoglobin concentration were only found with changes in the latter two laboratory indices. The significant correlations were weak and less important than the cumulative effect of unknown variables in determining haemoglobin level. The study shows that the routine clinical and laboratory parameters of disease activity in rheumatoid arthritis are of no value in predicting the haemoglobin concentration.