Trichothiodystrophy (TTD) is a rare hereditary multisystem disorder associated with defects in nucleotide excision repair (NER) as a consequence of mutations in XPD, XPB or TTDA, three genes that are all related to TFIIH, the multiprotein complex involved in NER and transcription. Here we show that all the mutations found in TTD cases, irrespective of whether they are homozygotes, hemizygotes or compound heterozygotes, cause a substantial and specific reduction (by up to 70%) in the cellular concentration of TFIIH. Intriguingly, the degree of reduction in the level of TFIIH does not correlate with the severity of the pathological phenotype, suggesting that the severity of the clinical features in TTD cannot be related solely to the effects of mutations on the stability of TFIIH. We have also measured TFIIH levels in cells in which different mutations in the XPD gene are associated with clinical symptoms not of TTD but of the highly cancer-prone disorder xeroderma pigmentosum (XP). We have found mild reductions (up to 40%) in TFIIH content in some but not all of these cell strains. We conclude that the severity of the clinical features in TTD patients and the clinical outcome of differentially mutated XPD proteins is likely to depend both on the effects that each mutation has on the stability of TFIIH and on the transcriptional activity of the residual TFIIH complexes.