Our study aimed to analyse clinical and metabolic characteristics of first degree relatives of patients with non-insulin-dependent diabetes mellitus (NIDDM) in Catalonia. Two hundred and five subjects (39.8 +/- 14.2 year-old, 61% women) were included in the study. An oral glucose tolerance test (OGTT) was performed, obtaining basal plasma glucose and insulin, in order to calculate, %B (HOMA beta cell function) and %S (HOMA insulin sensitivity). A 30.7% of subjects showed an abnormal glucose tolerance, either as impaired glucose tolerance (IGT) (20.5%) or as NIDDM (10.2%). Glycaemia after the OGTT (120 min) was independently determined by fasting glycaemia and age (R2 = 0.50; P < 0.001). As expected, subjects with normal glucose tolerance (NGT) were significantly younger than IGT and NIDDM subjects. The relatives with IGT and NIDDM display more features of syndrome-X when compared to NGT. Likewise, NGT relatives were less insulin sensitive and their basal insulin levels were higher when compared with a control group of subjects without familial history of NIDDM (log %S, 3.6 +/- 0.4 vs. 3.9 +/- 0.4; P = 0.000; log-insulin 2.4 +/- 0.4 vs. 2.1 +/- 0.6 mU/l; P < 0.02). In comparison with the general population, of any age group, NIDDM and IGT were more common in those subjects with a family history of NIDDM. Interestingly, the rates, of abnormal glucose tolerance in the 55-64 and > 64 year groups in the general population were similar to those seen in relatives two decades younger. Our study not only confirms a high prevalence of impaired glucose tolerance (IGT and NIDDM) in subjects with a family history of NIDDM, but also that these abnormalities can be detected at a very early age. Globally, this piece of information corroborates that special attention and precocious detection programs should be addressed to relatives of NIDDM patients.