Introduction Cerebral spinal fluid leak from durotomy is a well-known risk with spinal surgeries. The aim of this study is to identify the incidence of unrecognized incidental durotomy during posterior surgery for spinal metastases and its risk factors. Methods Participants comprised 75 patients who underwent posterior spine surgery for spinal metastases between January 2012 and December 2016. Cases with apparent durotomy noticed intraoperatively were excluded. Unrecognized durotomy was diagnosed as the presence of wide subcutaneous fluid retention on magnetic resonance imaging at least 3 months postoperatively. For comparison, 50 patients who underwent cervical laminoplasty due to cervical spondylotic myelopathy were examined using the same method. We also examined correlations between occurrence of durotomy and patient characteristics such as age, type of tumor, location of tumor (ventral or dorsal), extent of tumor, and history of radiotherapy before surgery. Results Unrecognized durotomy occurred in 21 cases of spinal metastasis (26.7%) and in 1 case of cervical spondylotic myelopathy (2%), representing a significant difference between groups. Age, type of tumor, location of tumor, extent of tumor, and history of radiotherapy before surgery did not correlate significantly with occurrence of durotomy. No local trouble was observed in durotomy cases, except in one case with subcutaneous local infection. Conclusions The incidence of unrecognized incidental durotomy is significantly higher during surgery for spinal metastases than that during surgery for degenerative disease.