We estimated the effect size of the strongest GWAS derived cystatin C SNP (major allele of rs13038305) on plasma cystatin C in the now completed MDC-CC-re-exam (n = 3,734) and thereafter examined the association between plasma cystatin C (403 cases of diabetes and 2665 controls) as well as rs13038305 (235 cases and 2425 controls) with incident diabetes. This evidence concerns the gene CST3 and diabetes mellitus.