IMPORTANCE: In Denmark, the incidence of malignant melanoma (MM) has doubled during the past 25 years, with an incidence of 29.5 and 31.7 per 100 000 person-years in 2012 for men and women, respectively. Understanding the nature of this increase in incidence is important to optimize prevention, early diagnosis, and treatment of in situ and invasive melanoma in Denmark. OBJECTIVE: To describe changes over time in the incidence and clinical and pathologic characteristics of in situ and invasive melanoma in Denmark from 1985 through 2012. DESIGN, SETTING, AND PARTICIPANTS: We used the official national Danish Melanoma Group database to describe all eligible, prospectively registered cases of in situ and invasive melanoma in Denmark from January 1, 1985, through December 31, 2012. Data analyses were performed from April 1, 2012, through January 31, 2013. MAIN OUTCOMES AND MEASURES: Estimated annual percentage changes (EAPCs) for men and women in European age-standardized incidence, age at diagnosis, and tumor region for in situ melanoma and MM. ForMMonly, melanoma type, Breslowthickness, ulceration, and mortality. RESULTS: We included 3299 cases of in situ melanoma and 20 760 cases of MM. The incidence (95%CI) of MMincreased by 4.5%(3.6%-5.3%) for men and 4.3%(3.5%-5.2%) for women, which was especially pronounced in patients older than 60 years (EAPCs, 5.8% [4.7%-6.8%] and 4.8% [3.8%-5.9%], respectively), in thin (Breslow thickness, <0.75 mm) melanoma (EAPCs, 6.6%[5.0%-8.2%] and 6.1% [6.0%-7.1%], respectively), and in superficially spreading MM(EAPCs, 5.2%[4.3%-6.2%] and 4.7%[3.9%-5.7%], respectively). We found no significant EAPC in the incidence of melanomas with Breslow thickness greater than 2.00mmin women, and relative ulceration rates (95%CI) declined in both sexes (EAPCs, -3.3%[-4.0% to -2.6%] in men and -3.4%[-4.0%to -2.8%] in women). More proximal tumor location occurred over time (P <.001). Incidence of in situ melanoma (95% CI) greatly increased (EAPCs, 14.0%[12.2%-15.8%] inmen and 11.6%[10.2%-13.2%] in women) with changes over time in age and region (defined by codes in the International Statistical Classification of Diseases, Tenth Revision) similar to those for MM. Mortality related toMMincreased in men (EAPC, 0.6%[0.1% to 1.2%]), whereas mortality in women (EAPC, -0.4%[-1.0%to 0.3%]) remained stable. CONCLUSIONS AND RELEVANCE: This study confirms a worldwide increase in melanoma incidence. Results may indicate the importance of secondary melanoma prevention in Denmark. Future efforts could intensify primary prevention aimed at young adults, adolescents, and children and maintain and target secondary prevention at the population older than 60 years.