Cardiac failure is a known complication of haemopoietic stem cell transplantation (HSCT) and is often difficult to diagnose as patients may have multiple medical problems. Since brain natriuretic peptide (BNP) is largely a hormone of cardiac ventricular origin and is released early in the course of ventricular dysfunction, we have examined the value of serial plasma BNP levels for detecting cardiac failure in patients undergoing cytotoxic conditioning for HSCT. Fifteen patients undergoing HSCT were evaluated (10 undergoing autologous HSCT; five undergoing allogeneic HSCT). BNP was measured by radioimmunoassay prior to therapy and weekly for 5 weeks. Seven patients had a significant rise in BNP level (above a previously established threshold of 43 pmol/l associated with cardiac failure), occurring 1-4 weeks post commencement of conditioning. In three of these patients, cardiac failure was subsequently diagnosed clinically 3, 9 and 23 days after a BNP level of 43 pmol/l had been detected. These three patients had the highest peak BNP levels for the group and in each case elevation in BNP level occurred for a period exceeding 1 week. Although numbers were relatively small, a BNP >43 pmol/l was significantly associated with the inclusion of high-dose cyclophosphamide in the preparative regimen (P = 0.02). BNP levels showed no relationship to febrile episodes. In conclusion, these results show that plasma BNP may be used as a marker for early detection of cardiac dysfunction in patients undergoing HSCT, particularly if levels are increased for periods exceeding 1 week. Measurement of BNP during HSCT may be helpful in patients at risk of cardiac failure, in complex clinical situations and in monitoring the cardiotoxicity of preparative regimens.