After observing a correlation between elevated serum lactate dehydrogenase (LDH) levels and good stem cell collections, retrospective data from 540 donors undergoing 650 stem cell apheresis procedures (87% autologous, 13% allogeneic) were studied to determine the correlation between preapheresis LDH levels and the absolute peripheral blood CD34 þ cell count (PBCD34). PBCD34 (1-1611/ll; median 40) correlated modestly with leukocytes (0.5-118.2 Â 10 9 /l; median 30.2) (r ¼ 0.16; P ¼ 0.00005) and poorly with platelets (16-660 Â 10 9 /l; median 131) (r ¼ 0.02; P ¼ 0.69). The correlation between LDH (64-1664 IU/l; median 310) and PBCD34 was very strong (r ¼ 0.54; Po10 À48 ). In multivariate regression analysis, serum LDH was the only factor correlating significantly with PBCD34. The correlation between serum LDH and PBCD34 was strong on the first day of collection (n ¼ 517; r ¼ 0.53; Po10 À37 ), weakened on the second day (n ¼ 74; r ¼ 0.37; P ¼ 0.0009) and disappeared beyond day 2 (n ¼ 59; r ¼ 0.09; P ¼ 0.49). PBCD34 was significantly higher (median 53 versus median 11; Po0.00001) when LDH was elevated (n ¼ 511) compared to when LDH was normal (n ¼ 139). The relationship between serum LDH and PBCD34 was strong for autologous (r ¼ 0.54) as well as for allogeneic (r ¼ 0.41) collections. Our data suggest that it is reasonable to assume good stem cell mobilization and start apheresis if the LDH is elevated.