Human DNA polymerase eta (Pol eta) modulates susceptibility to skin cancer by promoting DNA synthesis past sunlight-induced cyclobutane pyrimidine dimers that escape nucleotide excision repair (NER). Here we have determined the efficiency and fidelity of dimer bypass. We show that Pol eta copies thymine dimers and the flanking bases with higher processivity than it copies undamaged DNA, and then switches to less processive synthesis. This ability of Pol eta to sense the dimer location as synthesis proceeds may facilitate polymerase switching before and after lesion bypass. Pol eta bypasses a dimer with low fidelity and with higher error rates at the 3' thymine than at the 5' thymine. A similar bias is seen with Sulfolobus solfataricus DNA polymerase 4, which forms a Watson-Crick base pair at the 3' thymine of a dimer but a Hoogsteen base pair at the 5' thymine (ref. 3). Ultraviolet-induced mutagenesis is also higher at the 3' base of dipyrimidine sequences. Thus, in normal people and particularly in individuals with NER-defective xeroderma pigmentosum who accumulate dimers, errors made by Pol eta during dimer bypass could contribute to mutagenesis and skin cancer.