1. Serial estimations of the diffusing capacity for carbon monoxide, with a standard single-breath technique, were used to assist the monitoring of disease activity in patients at risk from intrapulmonary haemorrhage. 2. A reversible rise in diffusing capacity for carbon monoxide per unit alveolar volume (DLCO/VA) of 50% or more above baseline values was detected on 61 occasions and in the diffusing capacity for carbon monoxide (DLCO) alone on 45 occasions in 39 patients. 3. Concurrent with these rises in DLCO/VA or DLCO, two or more traditional indicators of intrapulmonary haemorrhage (haemoptysis, abrupt fall in haemoglobin concentration, chest X-ray opacities) were found on 47 occasions. 4. In the appropriate clinical context, acute reversible rises in DLCO/VA or DLCO reflect active intrapulmonary haemorrhage.