This study surveys the extent and severity of haematological and biochemical abnormalities which occurred in 265 patients with pulmonary tuberculosis, and records the haematological changes that occur with treatment. Anaemia was present in 60 per cent of patients, more frequently in males than in females. Leucocytosis with neutrophilia occurred in 40 per cent, lymphopenia in 17 per cent and monocytopenia in 50 per cent. Platelet count and erythrocyte sedimentation rate were elevated in 52 and 80 per cent respectively. Bone marrow aspiration and trephine biopsy were of limited diagnostic value. Ferritin and vitamin B12 levels were increased in 94 and 57 per cent of subjects respectively whilst serum and red cell folic acid were within normal limits in 83 per cent. The frequency of the important biochemical changes were hyponatraemia (43 per cent) and hypoalbuminaemia (72 per cent); alkaline phosphatase, aspartic transaminase and lactic dehydrogenase levels were elevated in approximately a third of patients possibly due to unsuspected dissemination. There was a close correlation between the acid-fast bacilli in sputum and abnormal values, particularly those of body weight, haemoglobin, platelet count, white cell count and erythrocyte sedimentation rate. Failure of these indices to return to normal was invariably associated with persistent excretion of acid-fast bacilli. We have shown that haematological and biochemical abnormalities in pulmonary tuberculosis are common and may be valuable aids to diagnosis. Some haematological markers also reflect response to treatment.