We investigated the environmental and personal risk factors associated with the development of basal cell carcinoma (BCC). This retrospective cohort study included a total of 997 patients whose diagnosis was confirmed by histopathologic examination between 2007 and 2014. A control examination was performed in 363 of these patients, who were accessed via telephone. A total of 1151 tumors were detected in 997 patients. During their follow-up, 13% of them developed subsequent tumors. The risk of developing subsequent tumor was 2.7-fold higher in patients with multiple BCCs at the time of diagnosis than those with single BCC. Multiple BCCs tended to develop in older patients and men. The risk of developing multiple BCCs was increased in patients with a history of BCC, skin type 1 or 2, and chronic sun exposure (sun exposure of >500 weeks, a high photoaging score [[>= 30], and the presence of actinic keratosis). We concluded that chronic sun exposure may increase the risk of developing multiple BCCs. These data also indicate that cumulative sun exposure is as important for developing BCC as for squamous cell carcinoma. (C) 2017 Elsevier Inc. All rights reserved.