Background: Anti-PD-1/PD-L1/CTLA4 agents have revolutionized the treatment landscape of many cancer types, resulting in durable responses. With recent approvals in the adjuvant setting, the long-term consequences of immune checkpoint inhibitors (ICIs) such as the impact on fertility are of increasing clinical relevance. The impact of ICI therapy on fertility is poorly understood. Based on published data, we hypothesize that patient fertility may be affected by ICIs via several mechanisms, including: endocrine dysfunction, direct impact on gametogenesis, or autoantibody formation. Methods: We designeda prospective, multi-center translational study that aims to identify the effects of ICIs on fertility. We aim to recruit 88 patients over 3-4 years. Participants of child-bearing age as defined by study protocol who are due to start ICIs for any solid tumour malignancy, will be eligible. The primary endpoint of the study is a change in anti-Müllerian hormone level and antral follicle count in females, and a change in sperm concentration, motility, morphology in males measured before and serially during ICI therapy (up to 24 months). Secondary endpoints include analysis of FSH, LH, testosterone levels, the proportion of patients with a persistent change in any one reproductive parameter as assessed at follow-up visits, frequency of oligo/amenorrhoea in females and immunologic changes in seminal fluid in males. Conclusions: ICIs have led to a significant improvement in the long-term survival in selected patients with cancer. This study will elucidate the effects of ICIs on both male and female fertility over time, and provide useful information to guide patients and their care teams regarding long-term effects of ICIs on fertility. Recruitment is ongoing.