This study was undertaken to determine the prevalence of diabetes mellitus (DM), insulin-dependent diabetes mellitus (IDDM), noninsulin-dependent diabetes mellitus (NIDDM) and impaired glucose tolerance (IGT) in different areas of Saudi Arabia. A household survey was conducted in 34 different areas of Saudi Arabia. Demographic data and medical history were taken and filled. Fasting and two-hour "post-glucose load" blood samples were collected from 23,493 Saudi males and females and blood glucose was estimated immediately. The diagnoses of DM and IGT were made based on the criteria of the World Health Organization (WHO). Diabetic patients were subgrouped as IDDM and NIDDM on the basis of age of onset and mode of treatment. In the overall group (two to 70 years), the prevalence of IDDM, NIDDM and IGT was 0.193%, 5.503% and 0.498% in the Saudi males and 0.237%, 4.556% and 0.900% in the Saudi females. When grouped on the basis of age, there were 8762 children (<14 years). Of these children, 15 (0.171%) had IDDM and 13 (0.148%) had maturity onset diabetes of the young (MODY). The prevalence of IGT in this group was 0.250%. In the 14-70-year age group, the prevalence of IDDM, NIDDM and IGT was 0.239%, 9.50% and 0.717% in the males and 0.248%, 6.820% and 1.347% in the females, respectively. A significant increase (P<0.001) was obvious in the age group >30 years, where the prevalence of NIDDM and IGT rose to 17.32% and 1.30% in the males and 12.18% and 2.2% in the females, respectively. IDDM showed a slight decrease in those over the age of 60 years. These results place Saudi Arabia among the countries that have a high prevalence for DM and a moderate risk for IGT. In light of these findings, it is clear that carefully planned programs are needed to achieve control and prevention of diabetes mellitus in Saudi Arabia.