We studied the prevalence of diabetes mellitus diagnosed in the Avila Health Care region, Spain, based on the prescribed daily dosage (PDD) of insulin (corrected in function of insulin loss (PDDc)) and sulphonureas, from a sample representative of prescribing physicians (A, n = 48) and pharmacies (B, n = 25) in our region. The amounts of the PDDcs of insulin sold during 1989 per 1000 inhabitants/day were 6.06 and 6.20, respectively for the two samples (A and B). The figures for PDDs of sulphonurea/1000 inhabitants/day were 16.25 and 19.16, respectively. We determined the proportion that diabetic patients on diet alone represented in relation to non-insulin treated diabetic patients who visited the only specialized clinic for diabetic patients in our province in 1989. These patients accounted for 40.83% of non-insulin treated diabetic patients. The patients being treated with biguanides or with a combination of anti-diabetic drugs accounted for less than 1% of those treated in the specialized clinic. Taking this into account, the prevalence of diabetes mellitus in Avila was 3.35% (C.I. 95%: 3.10-3.59), according to the sample A and 3.85% (C.I. 95%: 3.50-4.31) according to the sample B. In addition, we studied the hospital admission records during three consecutive years in order to find out the incidence of type 1 insulin-dependent diabetes mellitus (IDDM) below 15 years of age in the Avila Health Care region of Spain. All prescribing physicians that did not work in the Hospital (197 in the rural area and 18 in the urban area) were used as the secondary source for validation of case ascertainment.(ABSTRACT TRUNCATED AT 250 WORDS)