Background: Surveillance of childhood cancer incidence is informative for etiologic research and health policy. However, high-quality data covering several decades of virtually complete cancer diagnosis in children is sparse. Methods: Incident cases of childhood cancer (0–19 years at diagnosis), classified according to Birch and Marsden’s International Classification of Childhood Cancer, first edition (ICCC-1), were identified in the Danish Cancer Registry and used to calculate age-standardized incidence rates (ASRs) and estimated annual percentage change (EAPC) separately for 1943–1977 (early period) and 1977–2014 (recent period). Results: During 1943–2014, 15,184 childhood cancer cases were reported. The ASR for any cancer was 13.0 per 100 000 person-years in the early period (EAPC 0.55%; 95% CI 0.30–0.80) and 17.7 per 100 000 person-years in the recent period (EAPC 1.16%; 95% CI 0.96–1.36). In both periods, the increasing trend was seen in both boys (EAPC 0.69%; 95% CI 0.43–0.96/EAPC 0.96%; 95% CI 0.75–1.17) and girls (EAPC 0.37%; 95% CI −0.01–0.75/EAPC 1.41%; 95% CI 1.11–1.72) and in children aged 0–14 years (EAPC 0.53%; 95% CI 0.26–0.80/EAPC 0.86%; 95% CI 0.64–1.08) and 15–19 years (EAPC 0.60%; 95% CI 0.19–1.02/EAPC 1.97%; 95% CI 1.67–2.28). Increasing trends were observed for all main diagnostic groups. Conclusions: The incidence of childhood cancer in Denmark has increased since the 1940s, especially since 1977 and in older children. In recent years the increase has been most pronounced among girls.