Abstract: Sibling recurrence risk ratio (λS), defined as the ratio of risk of disease manifestation in siblings of probands compared with risk of disease in general population, is an extensively used measure of familial aggregation. A λS>1 is suggestive of familial aggregation. To assess the extent of familial clustering according to parental history of type 2 diabetes mellitus, the sibling recurrence risks (KS) and the sibling recurrence risk ratios (λS) were estimated in a randomly selected sample of 275 subjects with type 2 diabetes mellitus. A total of 325 out of 1125 siblings were affected giving an overall KS of 28.90% (95%CI: 26.21%-31.51%) and a λS of 2.31x (95%CI: 2.09x-2.50x) which is suggestive of a complex aetiology involving both genetic factors and environmental triggers. The KS and the λS values were elevated in families with one or two diabetic parents indicating that susceptibility to type 2 diabetes mellitus is transmitted primarily through an affected parent. The risk varied with respect to the status of the probands’ parents with KS (49.21%; 95%CI: 43.04%-55.38%) and λS (3.94x; 95%CI: 3.41x-4.43x) when both parents were affected being the highest reflecting a predominant influence of the predisposing genetic factors. The λS was found to be significantly higher (Z = 2.05; p = 0.04) when the affected parent was the mother (2.59x; 95%CI: 2.20x-3.06x) rather than the father (1.81x; 95%CI: 1.31x-2.31x) indicative of an excess of maternal transmission of type 2 diabetes mellitus. This is the first study on sibling recurrence risk ratio estimates for type 2 diabetes mellitus from India.