We assessed the incidence of diabetes mellitus (DM) in patients with heterozygous familial hypercholesterolemia (HeFH) and familial combined hyperlipidemia (FCH) treated with statins. Participants (n = 280) of mean age 59 ± 5 years were included (90 patients with HeFH, 112 patients with FCH, and 78 aged-matched participants). The median statin intensity treatment product (statin intensity in arbitrary equivalence units × duration of statin therapy in months) was 119 and 85 for patients with HeFH and FCH, respectively, at 10-year follow-up. The incidence of DM was significantly lower in patients with HeFH compared to the patients with FCH (2% vs 20%) and the reference group (2% vs 17%) during the 10-year follow-up period (all Ps < .001). Impaired fasting blood glucose at entry ( P < .001) and central obesity ( P = .02) were the only independent predictors of DM. The incidence of DM was significantly lower in older patients with HeFH compared to either aged-matched patients with FCH or individuals not receiving statins. Statins did not increase risk of DM in aging patients with FCH. These findings have implications, given the importance of high-intensity statin therapy for prevention of cardiovascular events, especially in patients with HeFH, a population with high cardiovascular risk.