Trichothiodystrophy (TTD) is a rare genetic disease associated in approximately 50% of patients with DNA repair deficiency analogous to that found in xeroderma pigmentosum group D (XP-D) patients. Although XP-D patients exhibit a very high level of skin cancer on sun-exposed parts, TTD is not associated with cancer. We analysed UV-induced mutations in TTD cells and compared them to data in XP-D in order to determine if the molecular mechanisms of mutagenesis can explain the discrepancies between these two syndromes. We first immortalized a fibroblast TTD line with an ori(-)-SV40 plasmid. To investigate the kinds of mutations induced in TTD cells, we used an UV-irradiated (at 254 nm) shuttle vector carrying the supF tRNA gene as a target. We compared our data with those published by others with the same pZ189 vector in normal and XP-D fibroblast lines (Bredberg et al., Proc. Natl. Acad. Sci. USA, 83, 8273-8277; Seetharam et al., J. Clin. Invest., 80, 1613-1617). The frequency of mutants increased linearly with UV dose and the slope was > 4 times steeper in TTD cells than that observed in normal cells. The mutation frequency was almost identical between XP-D and TTD cells. Sequence analysis of the supF tRNA gene showed that 96% of mutations obtained in TTD cells are base substitutions. Single base substitutions were found in 62% of mutants in TTD cells while they corresponded to 86% in XP-D cells. The frequency of multiple mutations in TTD cells (26%) was similar to that in normal cells (27%) and much higher than that in XP-D cells (9%). Despite the fact that the same gene is mutated in TTD and XP patients, the molecular characteristics of mutagenesis are not identical. The fact that the frequency of mutations in TTD and XP cells are similar shows that a high level of UV-induced mutations is therefore not always directly related to cancer-proneness. Other factors such as catalase activity and immuno-surveillance may intervene in cancer incidence.