Accelerated radiotherapy has the potential to increase local control of rapidly growing tumours. To determine the necessary time interval for complete repair of sublethal damage in normal tissue in a clinical situation, we have compared the acute and late skin reactions with 8 and 24 h between fractions, using the same dose per fraction and total dose. Forty-nine breast cancer patients participated in this study, and received bilateral parasternal irradiation to 50 Gy with 2 Gy per fraction as part of their adjuvant postoperative radiotherapy. The time interval between daily fractions was always 8 h on the left field and 24 h on the right, and the total treatment time was 2.5 and 5 weeks, respectively. The acute endpoint was erythema, measured by reflectance spectrophotometry and an acute reaction score for erythema and desquamation. The late endpoint was telangiectasia, scored on an arbitrary scale. The results have also been compared with those in a previously treated group of patients with 4 and 24 h between fractions. The degree of acute reactions was decreased with an 8-h interval compared with 24 h between fractions with the peak acute score as endpoint; no difference was seen with the peak reflectance measurements. The maximal expression occurs approximately 1 week earlier with the accelerated schedule, possibly as a consequence of the reduction of the treatment time. The pattern of the acute reaction for 8 h between fractions is similar to that for 4 h.(ABSTRACT TRUNCATED AT 250 WORDS)