Background Statins increase the risk of new-onset type 2 diabetes mellitus. We aimed to assess whether this increase in risk is a consequence of inhibition of 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR), the intended drug target. Methods We used single nucleotide polymorphisms in the HMGCR gene, rs17238484 (for the main analysis) and rs12916 (for a subsidiary analysis) as proxies for HMGCR inhibition by statins. We examined associations of these variants with plasma lipid, glucose, and insulin concentrations; bodyweight; waist circumference; and prevalent and incident type 2 diabetes. Study-specific effect estimates per copy of each LDL-lowering allele were pooled by meta-analysis. These findings were compared with a meta-analysis of new-onset type 2 diabetes and bodyweight change data from randomised trials of statin drugs. The effects of statins in each randomised trial were assessed using meta-analysis. Findings Data were available for up to 223 463 individuals from 43 genetic studies. Each additional rs17238484-G allele was associated with a mean 0.06 mmol/L (95% CI 0.05-0.07) lower LDL cholesterol and higher body weight (0.30 kg, 0.18-0.43), waist circumference (0.32 cm, 0.16-0.47), plasma insulin concentration (1.62%, 0.53-2.72), and plasma glucose concentration (0.23%, 0.02-0.44). The rs12916 SNP had similar effects on LDL cholesterol, bodyweight, and waist circumference. The rs17238484-G allele seemed to be associated with higher risk of type 2 diabetes (odds ratio [OR] per allele 1.02, 95% CI 1.00-1.05); the rs12916-T allele association was consistent (1.06, 1.03-1.09). In 129 170 individuals in randomised trials, statins lowered LDL cholesterol by 0.92 mmol/L (95% CI 0.18-1.67) at 1-year of follow-up, increased bodyweight by 0.24 kg (95% CI 0.10-0.38 in all trials; 0.33 kg, 95% CI 0.24-0.42 in placebo or standard care controlled trials and 0.15 kg, 95% CI 0.39 to 0.08 in intensive-dose vs moderate-dose trials) at a mean of 4. 2 years (range 1.9-6.7) of follow-up, and increased the odds of new-onset type 2 diabetes (OR 1.12, 95% CI 1.06-1.18 in all trials; 1.11, 95% CI 1.03-1. 20 in placebo or standard care controlled trials and 1.12, 95% CI 1.04-1. 22 in intensive-dose vs moderate dose trials). Interpretation The increased risk of type 2 diabetes noted with statins is at least partially explained by HMGCR inhibition.