We report here on two siblings who show no clinical manifestations except for slight cutaneous photosensitivity and cutaneous pigmentation but have biochemical characteristics o Cockayne syndrome (CS). Fibroblasts derived DNA synthesis (UDS)_in these cells was at a normal level, recovery of RNA synthesis (RRS) after UV irradiation was severely depressed. Microinjection of bacteriophage T4 endonuclease V into the cells corrected RRS after UV irradiation to a level near normal. These results indicate that DNA repair of cyclobutane-type pyrimidine dimers is impaired in the cells and the biochemical characteristics are similar to these of CS cells. However, cell fusion complementation tests with CS group A and B cells resulted in correction of RRS after UV irradiation. Cell fusion with XP group A, B, D, F and G cells also corrected RRS after UV irradiation, and microinjection of cell extracts prepared from Kps3 cells corrected UDS in XP group C and E cells, indicating that the patients do not belong to any complementation group of XP or CS. These results suggest that the patients have a new UV-sensitive syndrome with a biochemical phenotype of CS.