The clinical and laboratory studies on the disorders discussed here have yielded considerable insight into the importance of DNA repair processes in man. That unrepaired or incorrectly repaired damage to DNA can lead to malignancy and compromise the proper development and functioning of the nervous and immune systems is a reasonable conclusion. Our understanding of the relationship between molecular events that mediate DNA metabolism and cytogenetic and cellular phenomena such as chromosomal aberrations, mutagenesis, transformation and carcinogenesis is in its infancy. Continued efforts to clarify this relationship will assist in understanding, predicting and, hopefully, even controlling the carcinogenic process in man. Finally, heterozygous carriers of AT and XP genes would seem to be at increased risk of developing common cancers and non-melanoma skin neoplasms, respectively. The further evaluation of this possibility and its contribution to the overall cancer burden would seem to be of high priority in the study of environmental carcinogenesis.