In this report we present the results of a population based survey designed to characterize the prevalence of hypercholesterolemia (HCL: total cholesterol > or = 240 mg/dl) in a low income urban area of Mexico City (The Mexico City Diabetes Study). In an area of 15,532 inhabitants, 3505 (22.57%) were found to be study eligible (all 35-64 year-old men and nonpregnant women residing in the area permanently). A home interview was obtained in 2813 (80.26%) subjects. A physical exam and oral glucose tolerance test was performed on 2282 (81.2% of the individuals that gave the interview and 65.1% of all study eligibles, 941 men and 1341 women). The crude prevalence of HCL for men in this group was 12.3% and for women 12.5%. Severe HCL (total cholesterol > or = 260 mg/dl) was found in 6.6% of men and 6.5% of women. In the older age group (55-64 years) prevalence in women reached 21.1%. Of the individuals with HCL this abnormality was previously diagnosed in only 15.9% of men and in 7.8% of women. Despite having been diagnosed, the abnormality remained untreated in all cases. HCL was associated with higher mean systolic blood pressure (in women), higher mean diastolic blood pressure (in men) and higher mean fasting and 2 h post-glucose load glycemia as well as post-challenge insulinemia in both sexes. Mean TG was found high in both groups (with and without HCL), but in subjects with HCL the values were significantly higher. We conclude that HCL is very common in this population. The majority of the cases remained undiagnosed and of the small fraction of subjects that have been diagnosed, virtually none is under medical care. It is necessary to design and implement a national program to reduce the impact of this serious health problem.