Late malignancies have been discussed as a potential risk for growth factor mobilized donors of hematopoietic stem cells. Little is known about the incidence and potential risk factors. This single center retrospective cohort study evaluated all HLA-identical sibling pairs with hematopoietic stem cell transplantation (HSCT) for a hematological malignancy, treated from 1974 to 2001 at the University Hospital of Basel. Three hundred eighteen pairs were identified, 291 donors (92%) could be contacted. Median observation time was 13.8 years (range 5-32 years). Sixteen (5%) donors had developed a total of 18 tumors, 17 recipients a secondary tumor. According to the age- and sex-adapted cancer incidence, 3.3 tumors in male and 6.8 in female donors were expected, 3 (relative risk (RR): 0.91, 95% confidence interval: 0.19-2.66) and 4 (RR: 0.58, 95% confidence interval: 0.16-1.48), respectively, were found in donors between 0 and 49 years. Between 50 and 69 years, 4.5 tumors in males and 4.8 in females were expected, 5 (RR: 1.11, 95% confidence interval: 0.36-2.59) and 6 (RR: 1.23, 95% confidence interval: 0.45-2.67), respectively, were observed. Tumors do occur in donors of hematopoietic stem cells at least at the rate as expected in a normal population; whether incidence exceeds expected rates needs to be determined in larger international cohorts.