Obesity is often accompanied by non-insulin-dependent diabetes mellitus (type 2), arterial hypertension, and hyperlipidaemia. The aim of this study was to evaluate whether duration of obesity is a risk factor for the appearance of type 2 diabetes, hypertension, and hyperlipidaemia. We studied 760 obese subjects, 207 of whom had normal glucose tolerance, 125 impaired glucose tolerance, and 428 type 2 diabetes; in addition, 560 had hypertension and 315 had hyperlipidaemia. At univariate analysis, passing from normal through impaired glucose tolerance to type 2 diabetes there was a progressive increase of age and of duration of obesity, hypertension and hyperlipidaemia. Compared to subjects without hypertension, hypertensive subjects were older, had a longer duration of obesity, a greater body mass index (BMI, kg/m2), and more frequently a family history of hypertension; they also more frequently showed impaired glucose tolerance and type 2 diabetes and hyperlipidaemia. Compared to subjects without hyperlipidaemia, hyperlipidaemic subjects were older, had a longer duration of obesity, and more frequently showed impaired glucose tolerance and type 2 diabetes, and hypertension. Diabetes, hypertension, and hyperlipidaemia were highly associated, as up to 80% of subjects with type 2 diabetes had hypertension, and more than 80% of hyperlipidaemic subjects had hypertension. Type 2 diabetes was less frequent than hypertension and hyperlipidaemia during the first 10 years of obesity, and progressively increased thereafter; in contrast the frequency of hypertension and of hyperlipidaemia increased only after 30 years of obesity. In 359 subjects undergoing an oral glucose tolerance test (168 with simultaneous determination of insulin release), increasing durations of obesity were accompanied by an increasing prevalence of type 2 diabetes, and in deterioration of glucose response, with no decrease in insulin release. At logistic regression analysis, age was a common risk factor for diabetes, hypertension, and hyperlipidaemia; duration of obesity and hyperlipidaemia were additional risk factors for diabetes; family history of hypertension, BMI and hyperlipidaemia were additional risk factors for hypertension, as were impaired glucose tolerance or diabetes, and hypertension for hyperlipidaemia. These data indicate that duration of obesity is a risk factor for type 2 diabetes, and emphasize the importance of preventing obesity in young subjects.