Individualization of radiation doses is presumed to result in better radiotherapy outcome. Success rate in measuring radiosensitivity is probably the most limiting factor for present radiosensitivity assays to be introduced into clinical routine. To find a simpler predictive parameter, we compared the radiosensitivity of dermal fibroblasts and head and neck squamous cell carcinoma (SCC) cell lines established from the same individuals. The radiosensitivity was tested using the clonogenic 96-well plate assay. The surviving fraction at 2.0 Gy (SF2) was determined, as well as the mean inactivation dose (AUC) of cancer cells. SF2 of SCC cell lines and skin fibroblasts were 0.25-0.44 and 0.11-0.43, respectively. AUC of SCC cells was 1.4 -2.1 Gy. Dermal fibroblasts were more radiosensitive than SCC cells in 14 of 15 cases. In 1 patient (UT-SCC-8), cancer cells were found to be more radiosensitive than corresponding dermal fibroblasts. There was a clear tendency to a correlation between radiosensitivities of these 2 cell types, but statistical significance was reached only when the data of UT-SCC-8 was excluded. In our material, the intrinsic radiosensitivity of head and neck SCC cells could in most cases be predicted from the intrinsic radiosensitivity of dermal fibroblasts established from the same individual. Surgery and radiation therapy are the cornerstones in the treatment of head and neck squamous cell carcinoma (HNSCC). In most cases these treatment modalities are combined in order to achieve better therapy response. In practice, the given dose of radiation is, however, limited by acute and late side effects. Not only is the prescribed dose a limiting factor in the normal tissue for the treatment but also the fractionation used. In breast cancer patients, identical doses of external radiotherapy have been shown to cause a varying grade of acute and late dermal side effects, and it has been suggested that the radiosensitivity of all cells is genetically determined in each individual. 1,2 In genetic disorders, ataxiatelangiectasia, e.g., the patients are highly radiosensitive both in vitro and in vivo. In theory, in genetically sensitive individuals a lower radiation dose can be given in order to prevent side effects, but this will not necessarily result in a reduced probability of tumour control. 3 It has been estimated that a subgroup of patients with radioresistant fibroblasts might be able to tolerate perhaps up to 40% larger doses without experiencing a significant increase in severe late skin complications. 4 The in vitro radiosensitivity of skin fibroblasts has most often been regarded to correlate with the clinical expression of late skin reactions. [5][6][7][8] On the other hand, a lack of correlation between the intrinsic radiosensitivity of dermal fibroblasts and early skin reactions has been found in the majority of studies. 6,8 -11 Experimental studies have suggested that the in vitro radiosensitivity of tumour cells, estimated by the fraction of surviving cells after a radiation dose of 2.0 Gy (SF...