AIMS/HYPOTHESIS: South Asian individuals have an increased prevalence of type 2 diabetes, but little is known about the development of glycaemic traits in this ethnic group. We compared age-related changes in glycaemic traits between non-diabetic South Asian and white participants. METHODS: In a prospective British occupational cohort with 5-yearly clinical examinations (n = 230/5,749 South Asian/white participants, age 39-79 years at baseline), age-related trajectories of fasting glucose (FG) and 2 h post-load glucose (PLG), log-transformed fasting insulin (FINS) and 2 h post-load insulin (PLINS), HOMA insulin sensitivity (HOMA2-%S) and HOMA insulin secretion (HOMA2-%B) were fitted for South Asian and white individuals who remained free of diabetes between 1991 and 2009. RESULTS: In sex-adjusted multilevel models, FG was stable in white participants but increased with age in South Asians (0.12 [SE = 0.04] mmol/l per decade). PLG, FINS and PLINS levels were lower among white participants (by 0.271 [SE = 0.092] mmol/l, 0.306 [SE = 0.046] log pmol/l, 0.707 [SE = 0.059] log pmol/l at age 50, respectively) compared with South Asians, although their age-related trajectories were parallel. HOMA2-%S was higher (0.226 [SE = 0.038] at age 50) and HOMA2-%B lower (by 0.189 [SE = 0.026] at age 50) among white than South Asian participants. The age-related decline in HOMA2-%S was similar in these groups, but the age-related increase in HOMA2-%B was greater in white participants (0.04 [SE = 0.02] per decade). This difference was explained by obesity, lifestyle and social status. CONCLUSIONS/INTERPRETATION: Findings from a diabetes-free population suggest an inadequate pancreatic beta cell reserve in South Asians, as a significantly steeper age-related increase in FG was observed in this ethnic group compared with white individuals.