BACKGROUND:Merkel cell carcinoma (MCC) is an aggressive neuroendocrine tumour of the skin that has been associated with a new tumour virus, the MCC polyomavirus. METHODS:To investigate whether MCC may have a shared aetiology with other cancers, we investigated the risk of second cancers after the diagnosis of MCC using the national cancer registries in Denmark, Norway and Sweden. RESULTS:The overall cancer incidence was increased among patients diagnosed with MCC compared with the general population in these countries (79 secondary cancers total, Standardized Incidence Ratio (SIR) 1.38 (95% confidence interval (CI): 1.10-1.72); 49 secondary cancer in females, SIR 1.7 (95% CI: 1.29-2.25); 30 secondary cancers in males and SIR 1.05 (95% CI: 0.73-1.5)). There were significantly increased incidence ratios for non-melanoma skin cancers (34 secondary cancers, SIR 8.35 (95% CI: 5.97-11.68)), melanoma of skin (6 secondary cancers, SIR 4.29 (95% CI: 1.93-9.56)) and laryngeal cancer (2 secondary cancers, SIR 9.51 (95% CI: 2.38-38)). The SIRs for these three cancer sites were also elevated on restricting the follow-up to cancers occurring at least one year after MCC diagnosis. CONCLUSIONS:Patients diagnosed with MCC are at increased risk of a second cancer, particularly, other skin cancers. Conceivable explanations include the impact of increased surveillance of the skin and shared causative factors, for example, ultraviolet light exposure or MCC polyomavirus infection.