The aims of our study were to determine the impact of a genetic diagnosis on diabetes treatment in UK individuals with GCK-, HNF1A- or HNF4A-MODY, and to identify clinical characteristics that predict successful management (i.e. HbA1c ≤58 mmol/mol [≤7.5%]) with no treatment in those with GCK-MODY or sulfonylureas in those with HNF1A and HNF4A-MODY at 2 years after genetic diagnosis. This evidence concerns the gene HNF1A and diabetes mellitus.