On the basis of 55 years of continuous cancer registration in Denmark, we present cancer incidence rates, time trends and birth cohort analyses for persons aged 0–34 years. The group of 40,750 cancer patients showed a substantial over-representation of males aged 1–24 years. The cancer pattern among young (15–34 years) men was dominated by testicular cancer (35%), lymphomas (14%) and tumors of the brain (13%), while the pattern among young women was governed by invasive cervical cancer (19%), malignant melanoma (15%) and cancer of the breast (12%). In this age range, a positive time trend was seen after 1970, equivalent to average annual percentage increases of 1.9% for men and 1.8% for women, due mainly to markedly increasing trends for testicular cancer, malignant melanoma, brain tumors, thyroid cancer, skin carcinomas and skin sarcoma among men, and for brain tumors, non-Hodgkin lymphoma, malignant melanoma, skin carcinomas and thyroid cancer among women. We saw no clear time trend for breast cancer among women. The cancer pattern among children (0–14 years) was similar to that reported for other white populations. © 2005 Wiley-Liss, Inc.