Some evidence suggests that Ca and vitamin D supplements affect cancer risk; however, it is uncertain whether the effects are due to Ca, vitamin D or the combination. We investigated the effect of Ca supplements without co-administered vitamin D on cancer risk. Medline, Embase and the Cochrane Central Register of Controlled Trials, reference lists of meta-analyses and two clinical trial registries were searched for randomised, placebo-controlled trials of Ca supplements ( ≥ 500 mg/d), with ≥ 100 participants and duration >1 year. The lead authors of eligible trials supplied data on cancer outcomes. Trial-level data were analysed using random-effects meta-analyses and patient-level data using Cox proportional hazards models. A total of sixteen trials were eligible, six had no data available, ten provided trial-level data (n 10,496, mean duration 3·9 years), and of these, four provided patient-level data (n 7221, median duration 3·5 years). In the meta-analysis of trial-level data, allocation to Ca did not alter the risk of total cancer (relative risk 0·95, 95% CI 0·76, 1·18, P= 0·63), colorectal cancer (relative risk 1·38, 95% CI 0·89, 2·15, P= 0·15), breast cancer (relative risk 1·01, 95% CI 0·64, 1·59, P= 0·97) or cancer-related mortality (relative risk 0·96, 95% CI 0·74, 1·24, P= 0·75), but reduced the risk of prostate cancer (relative risk 0·54, 95 % CI 0·30, 0·96, P= 0·03), although there were few events. The meta-analysis of patient-level data showed similar results, with no effect of Ca on the risk of total cancer (hazard ratio 1·07, 95% CI 0·89, 1·28, P= 0·50). Ca supplements without co-administered vitamin D did not alter total cancer risk over 4 years, although the meta-analysis lacked power to detect very small effects, or those with a longer latency.