The short-term effects of radiotherapy on sperm chromosomes was evaluated in two Hodgkin patients. Sperm cytogenetics were analysed by using human-hamster cross-fertilization. Immediately after irradiation, most sperm metaphases from the first patient exhibited multiple rearrangements, whereas in the second one, the incidence of such abnormalities was only slightly increased. Multi-fragmented chromosomes and structural aberrations of the chromatid-type were observed, suggesting a specific immediate response.