OBJECTIVE High-density lipoprotein cholesterol (HDL-C) and apolipoprotein A-I (Apo A-I) are key cardiovascular risk factors, but whether the ratio of HDL-C/Apo A-I concentrations affects risk of cardiovascular disease (CVD) and other diseases is uncertain. To investigate whether HDL-C and Apo A-I concentrations and the ratio of HDL-C/Apo A-I affect risk of death from CVD, cancer and all causes. METHODS Data were analysed from an occupational cohort of 263,340 people between 2002 and 2009. Cox proportional hazards models were used to estimate HRs (and 95% CIs) for mortality using the sex-specific lowest quartiles of HDL-C, Apo A-I concentrations and HDL-C/Apo A-I ratio as the reference groups. METHODS 1012 participants died (median follow-up 4.2 years). There were no significant associations between HDL quartiles and all mortality outcomes. In contrast, there was a positive trend for the association across increasing HDL/Apo A-I ratio quartiles and mortality from CVD, cancer and all cause (p values for trends across quartiles=0.016, 0.001 and <0.001, respectively). The adjusted HRs for highest HDL/Apo A-I ratio quartile versus the lowest were 2.37 (95% CI 0.89 to 6.37) (CVD); 2.32 (95% CI 1.34 to 4.03) (cancer) and 1.87 (95% CI 1.32 to 2.66) (all-cause mortality). CONCLUSIONS These data show for the first time that an increased HDL-C/Apo A-I ratio may be a shared risk factor for CVD, cancer and all-cause mortality.