Importance: Previous randomized controlled trials have provided inconsistent results on the effect of vitamin D supplementation on cancer incidence. Objective: To determine if monthly high-dose vitamin D supplementation, without calcium, reduces cancer incidence and cancer mortality in the general population. Design: Randomized, double-blind, placebo-controlled trial, participants recruited from April 2011 to November 2012, follow-up until December 2015. Setting: Recruited mostly from family practices in Auckland, New Zealand. Participants: Community-resident adults, aged 50-84 years. Out of 47,905 adults invited from family practices, and 163 from community groups, 5,110 participants were randomized to vitamin D3 (n=2,558) or placebo (n=2,552). Two participants withdrew consent, and all others (n=5,108) were included in the primary analysis. Intervention: Oral vitamin D3, initial bolus dose of 200,000 IU, followed one month later by monthly doses of 100,000 IU, or placebo, for median of 3.3 years (range: 2.5–4.2 years). Main Outcomes and Measures: The post-hoc primary outcome was all primary neoplasms (invasive and in-situ), aside from non-melanoma skin cancers, diagnosed from randomization to stopping the study medication (31 July 2015). Secondary outcomes were all neoplasms: from randomization to 31 December 2015; from >12 months after randomization to both stopping the study medication and also to 31 December 2015; and fatal neoplasms from randomization to 31 December 2015.