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Study question Is the rate of fatherhood among men diagnosed with cancer in childhood and early adulthood different from men without cancer – have differences changed over time? Summary answer Men diagnosed with cancer had significantly reduced rates of fatherhood compared with undiagnosed men. Rates of fatherhood among the cancer survivors increased markedly over time. What is known already The number of children and young adolescents who survive cancer has steadily increased over the past decades, with a current 5-year survival rate of approximately 80%. Consequently, life circumstances after cancer have gained increasing importance, including the desire among survivors to have children and a family. MAR technologies to aid reproduction among cancer survivors have been developed, and fertility preservation is increasingly a topic being discussed before undergoing cancer treatment. But the potential for fertility preservation differs depending on age at diagnosis and type of cancer. Earlier studies have shown decreased fertility rates among survivors of childhood and adolescent cancer. Study design, size, duration This study is a national, register-based cohort study. Men diagnosed with cancer in childhood and early adulthood ( Participants/materials, setting, methods Fatherhood among the boys and young men diagnosed with cancer was compared with the age-matched comparison group in all statistical analyses. Cancer diagnoses were categorized as central nervous system (CNS), haematological cancers or solid cancers. Also, analyses were stratified by age at diagnosis (0–9, 10–19, 20–29 years) and year of diagnosis (1978–89, 1990–99, 2000–16). Death was incorporated as a competing risk in all analyses. Main results and the role of chance The study population consisted of 15,600 boys and young men diagnosed with cancer between 1978 and 2016 and 1,386,493 men in the age-matched comparison group. Men surviving CNS cancer had the lowest hazard ratio of fatherhood compared with the age-matched comparison group (HR = 0.64, 95% CI 0.57–0.73), followed by survivors of haematological cancers (HR = 0.90, 95% CI 0.82–0.98) while the highest chance of fatherhood was slightly increased among survivors of solid cancers (HR = 1.13, 95% CI 1.10–1.16). The hazard ratio of becoming a father increased over time. From the first decade to the last decade 30 years later, the hazard ratio of becoming a father increased for solid tumours (HR 0.76, 95% CI 0.72–0.80 to HR 1.07, 95% CI 0.96–1.19), haematological tumours (HR 0.60, 95% CI 0.51–0.71 to HR 0.97, 95% CI 0.76–1.23) and CNS tumours (HR 0.47, 95% CI 0.39–0.58 to HR 1.04, 95% CI 0.56–1.93) compared to the age-matched comparison group. Also, men diagnosed with cancer when aged 20–29 years more likely became fathers over time (HR 0.79, 95% CI 0.74–0.84 to HR 1.09, 95% CI 0.98–1.22). Limitations, reasons for caution The study was based on register data, and information was not available about the men’s fertility potential, whether they had a desire to have children and whether it was possible for them to find a partner. Also, information about fertility preservation, e. g. sperm freezing, could have provided additional insights. Wider implications of the findings: Information and education of male patients diagnosed with cancer about fertility preservation options, and chances to create their own family is crucial. Reassuringly, time trends showed more men with a previous cancer diagnosis becoming fathers in recent years than earlier, reflecting that survival and fertility preservation have improved over time. Trial registration number N/A