Mammalian somatic cells can be directly reprogrammed into induced pluripotent stem cells (iPSCs) by introducing defined sets of transcription factors. Somatic cell reprogramming involves epigenomic reconfiguration, conferring iPSCs with characteristics similar to embryonic stem (ES) cells. Human ES cells contain 5-hydroxymethylcytosine (5hmC), which is generated though the oxidation of 5-methylcytosine (5mC) by the TET family of enzymes. Here we show that 5hmC level increases significantly during reprogramming due to the activation of TET1. During this process, dynamic genome-wide 5hmC modification occurs across the genome with more modifications at telomere-proximal regions. Compared with hES cells, we found iPS cells tend to form large-scale (100kb-1.3Mb) aberrant reprogramming hotspots in subtelomeric regions, most of which display incomplete hydroxymethylation. Strikingly, these 5hmC aberrant hotspots largely coincide (>80%) with previously reported aberrant non-CG methylation regions. Our results suggest that 5hmC modification could play important roles during reprogramming to pluripotency, and contribute to the differences between iPSCs and hESCs. we generated comprehensive genome-wide profiles of 5hmC in somatic cells, iPS cell lines derived from a variety of origins, and multiple hES cell lines.