The diagnosis of diabetes subtypes in a busy clinic is usually based on the age at onset, phenotype and biochemical diagnostic criteria. Not surprisingly, the focus is predominantly on diagnosing type 1 and type 2 diabetes. More challenging for the physician is to think of monogenic diabetes, which results from gene mutations that reduce beta cell function. Often misdiagnosed as type 1 or type 2 diabetes, maturity-onset diabetes of the young (MODY) is the most common form of monogenic diabetes, accounting for 1-2% of all diabetes cases. A diagnosis of monogenic diabetes has important implications in treatment, the prediction of disease progression and screening of family members. We highlight two cases of monogenic diabetes, where the diagnosis had significant implications in all the above parameters. This is followed by a discussion on the glucokinase and hepatocyte nuclear factor-1 alpha MODY, which together constitute the most frequent cause of MODY syndromes in all populations.