ABSTRACT The nucleoside analog 5-aza-2’-deoxycytidine (5-aza-dC) is used to treat some hematopoietic malignancies. The mechanism of cell killing depends upon DNMT1, but is otherwise not clearly defined. Here we show that PARP1 forms covalent DNA adducts in human lymphoblast or fibroblasts treated with 5-aza-dC. Some adducts recovered from 5-aza-dC-treated cells have undergone cleavage by apoptotic caspases 3/7. Mapping of PARP1-DNA adducts, by a new method, “Adduct-Seq”, demonstrates adduct enrichment at CpG-dense genomic locations that are targets of maintenance methylation by DNMT1. Covalent protein-DNA adducts can arrest replication and induce apoptosis, and these results raise the possibility that induction of PARP1-DNA adducts may contribute to cell killing in response to treatment with 5-aza-dC.