During the years 1977-1986 five patients were referred to the Department of Plastic Surgery. University Hospital, Arhus, for treatment of carcinomas occurring in the skin on the sites previously treated with Grenz rays for benign dermatoses. Grenz ray treatment had been given for eight to nineteen years. The dose given, in one case, was very extensive. Otherwise a dose between 10,000 rad (100 Gy) and 29,300 rad (293 Gy) was given. The tumours were squamous cell carcinomas in four cases and basal cell carcinoma in one case. One person developed both squamous and basal cell carcinomas. The tumours behaved aggressively and in one young patient the dura and the parietal bone had to be replaced by tensor fasciae latae graft and a free latissimus dorsi flap. None of the patients had been exposed to other known carcinogens. Caution in applying Grenz ray treatment is stressed.