Objective: To determine the cumulative probability of hypertension in type 2 diabetic patients, and of diabetes in patients with essential arterial hypertension. Methods: A 15-years follow-up study of 308 subjects, 156 with type 2 diabetes and 152 with arterial hypertension. Time zero (T0) was the date in which the diabetes or the hypertension were established. The time in years from T0 up to diagnosis of hypertension in diabetics or diabetes in hypertensives was determined. The cumulative probability was calculated with the method of Kaplan and Meier. Results: The incidence of hypertension in the diabetics was 4.36 patient-year, and that of diabetes in hypertensives of 1.23 patient-year (p = < 0.00001; Cl95% 4.8-14.7). After the first 5 years, 40% of the diabetics had hypertension and 12% of the hypertensives developed diabetes; at 15-years follow-up, these figures were 71% and 21% respectively (p < 0.001). Conclusions: The probability of essential arterial hypertension in type 2 diabetes was significantly higher than the incidence of diabetes in hypertensive subjects. In both instances, the risk was greater for men than for women.