To characterize nucleotide excision repair properties of cells from trichothiodystrophy (TTD) patients genetically-related to the xeroderma pigmentosum (XP) group D, TTD skin fibroblasts from two unrelated patients (TTD1VI and TTD2VI) belonging to the TTD/XPD group were transformed with a plasmid containing SV40 large T antigen-coding sequences and some DNA repair properties, such as unscheduled DNA synthesis (UDS), UV-survival, in vitro repair synthesis of cell extracts and reactivation of UV-irradiated reporter plasmid were studied. Results showed that: a) both untransformed and transformed TTD cells present a reduced UV-survival, compared to wild-type cells, but at significantly less reduced levels than XP-D cells; b) reduced repair activities were detected in both TTD and XP-D transformed cells by using in vitro cell free extract repair and reactivation of UV-irradiated plasmid procedures, and these relative reduced extents correlated with respective UV-survival; c) surprisingly, near wild-type UDS levels were detected in TTD2VILas transformed cells at different passages after the crisis, suggesting a phenotypic reversion of this transformed cell line; d) fluoro-cytometric analysis of TTD2VILas cells revealed a strong increase of a cell population containing a DNA amount more than twice as high than that of untransformed cells; finally, e) when UDS data were normalized to the DNA content in TTD2VILas cells, it appeared that the repair efficiency was only slightly higher than in untransformed cells. This implies that in transformed cells DNA repair properties should be evaluated, taking into account additional parameters. We obtained an immortalized TTD cell line which maintains DNA repair properties similar to those of parental untransformed cells and may be used to characterize the TTD defect at genetic, molecular and biochemical levels.