Objective: To investigate if the occurrence of subcutaneous fibrosis after radiotherapy in an unselected group of breast cancer patients is related to cellular radiosensitivity of skin fibroblasts as measured in a clonogenic assay. Materials and methods: An in vitro colony-forming assay of normal fibroblast radiosensitivity was applied to primary skin biopsies from 31 breast cancer patients who received post-mastectomy radiotherapy with large doses per fraction (2.7-3.9 Gy) more than 10 years earlier. Three clinical normal-tissue endpoints were assessed. Two late endpoints, subcutaneous fibrosis and telangiectasia, were evaluated in three treatment fields by a single experienced clinician. In addition, skin erythema had been assessed at the end of treatment by members of the staff and junior staff. From previous analyses of normal tissue response, individual clinical radiosensitivity could be assessed as 'excess risk' of each of the three reactions. This was defined as the difference between the actual observed response in the patient and the expected response estimated from individual treatment characteristics in a linear quadratic (LQ) mixture model and, for the two late endpoints, with correction for the follow-up time. This clinical radioresponsiveness was compared with the in vitro radiosensitivity of the skin fibroblasts. To this end, the fractions of colony-forming cells after graded single doses were fitted by an LQ survival curve using non-linear and linear regression from which the surviving fraction at 3.5 Gy (SF3.5) was estimated. Assessment at 3.5 Gy was chosen to reflect the fraction size during clinical radiotherapy. Results: A statistically significant variability of in vitro radiosensitivity between patients could be detected for both SF2 (P = 0.0095) and SF3.5 (P = 0.0008). A significant correlation was observed between SF3.5 and excess risk of fibrosis (r(s)= -0.46, P = 0.009) while no association was found between fibroblast radiosensitivity and either the occurrence of severe skin telangiectasia or the acute endpoint skin erythema. Conclusion: These results suggest that variability in the occurrence of subcutaneous fibrosis, but not telangiectasia or erythema, after radiotherapy is partly accounted for by differences in cellular radiosensitivity of normal skin fibroblasts.