Twenty-two consecutive patients with a median age of 11 years (range 3-23) underwent 103 leukaphereses for collection of peripheral stem cells. They suffered from various solid tumors and hematologic malignancies. Our aim was to collect at least 2.5 x 10(4) myeloid committed stem cells (CFU-GM) per kg body weight. This stem cell number has reliably resulted in rapid and sustained hematopoietic reconstitution after autografting in our institution. Peripheral stem cell mobilization was induced by myelosuppressive therapy alone. If the stem cell collections were started at the first platelet rise and were continued during the following 8 days, the stem cell yield of one procedure was high - median 1.02 x 10(4) CFU-GM/kg body weight (range 0.02-13.3). In 69 such 'well-timed' collections, a median of only 4 leukaphereses (range 2-6) were needed to collect high numbers of CFU-GM. The yield from leukaphereses carried out at any other time was unsatisfactory. In 34 'random' collections, a median of only 0.27 x 10(4) CFU-GM/kg body weight (range 0.01-1.11) was obtained in one procedure. In 69 well-timed collections the CFU-GM yield depended on the absolute number of circulating leukocytes (r = 0.369, p = 0.002) and mononuclear cells (r = 0.476, p less than 0.001). The median daily increment of leukocytes to the day of apheresis proved to be an excellent parameter for the prediction of the yield and could be used to select the best days for leukaphereses (r = 0.403, p = 0.002). These findings may help to make the collection of blood-derived stem cells more efficient in pediatric patients.