We report the long-term follow-up of 5 patients (4 kidney, 1 heart recipients) having a pretransplant monoclonal gammopathy of undetermined significance (MGUS). The follow-up of MGUS before transplantation was 41.2 +/- 40.7 months (range 2-108). The monoclonal component isotype was IgG-kappa in 3 cases and IgA-lambda in 2. The pretransplant level of the monoclonal component was 11.1 +/- 4.8 g/l (range 4-15.6). The transplant recipients who had MGUS were older than our other transplant patients. All but one of them received ciclosporine A. They did not experience more rejection or infectious complications than the others. Their posttransplant follow-up ranged from 3 to 9 years. The monoclonal component level remained stable in 2 patients but increased in 3 (33-225%). This was not correlated with bone marrow plasmocytosis. Two patients developed smoldering myeloma indicated by bone marrow immunochemistry studies which showed monomorph monoclonal plasma cells; nevertheless, they did not have cytopenia or bone lytic lesions. This percentage is higher than in the Kyle study but we cannot assume that it is due to ciclosporine A since our study sample is too small. In conclusion, MGUS is not a contraindication to organ transplantation.