Lymphocytes of healthy volunteers (n = 24) and of tumour patients (n = 30, 18 of 
whom had experienced severe side-effects) were irradiated with x-rays in vitro. 
DNA damage was analysed after 0.25-2 Gy and DNA repair after 2 Gy, and 
quantification of both endpoints was done by the comet assay. The individual 
differences in radiation-induced DNA damage as well as in the repair kinetics 
were observed to be striking for both healthy donors and tumour patients. After 
a repair time of 3 h, following 2 Gy x-irradiation, some of the healthy 
volunteers showed no residual DNA damage at all in their lymphocytes, whereas 
others revealed about 30%. There was no indication that our results were 
affected by either age, gender or smoking habits. Slow repair kinetics and high 
amounts of residual damage were characteristic for many but not all tumour 
patients who had experienced severe side-effects in their normal tissues during 
or after radiotherapy (n = 18). Our conclusion is that those individuals showing 
poor DNA repair characteristics in the lymphocytes following in vitro 
irradiation, have a high probability of being radiosensitive. The opposite 
conclusion is not necessarily true: if repair is effective, this does not mean 
that the individual is radioresistant, because factors other than impaired 
repair may cause radiosensitivity.