Biological consequences and physical complaints were compared for donors randomly assigned either to blood stem cell (BSC) or bone marrow (BM) donation. In the period 1994-1999, 61 consecutive donors were included. The BSC donors were given G-CSF 10 microg/kg s.c., daily during 5 days before the first leukapheresis. Nineteen donors had one leukapheresis, 10 required two and one donor needed three leukaphereses in order to reach the target cell number of 2 x 10(6) CD34(+) cells/kg bw of the recipient. A median platelet nadir of 102 x 10(9)/l was reached shortly after the last leukapheresis. Three weeks post harvest, 17 of 30 BSC donors had a mild leukopenia. Six had a leukopenia lasting more than a year before returning to normal values. Both groups were monitored prospectively through a standardised questionnaire completed by the donors. BSC donation was significantly less burdensome than BM donation and was preferred by the donors. The short-term risks of BSC mobilisation and harvest seem negligible. The potential long-term effects of G-CSF are unresolved and the donors must be followed closely.