BACKGROUND: While most of the global burden from chronic diseases, and especially vascular diseases, is now borne by low and middle-income countries, few large-scale epidemiological studies of chronic diseases in such countries have been performed. METHODS: From 1998-2004, 52 584 men and 106 962 women aged > or =35 years were visited in their homes in Mexico City. Self reported diagnoses of chronic diseases and major disease risk factors were ascertained and physical measurements taken. Age- and sex-specific prevalences and means were analysed. RESULTS: After about age 50 years, diabetes was extremely common - for example, 23.8% of men and 26.9% of women aged 65-74 reported a diagnosis. By comparison, ischaemic heart disease was reported by 4.8% of men and 3.0% of women aged 65-74, a history of stroke by 2.8% and 2.3%, respectively, and a history of cancer by 1.3% and 2.1%. Cancer history was generally more common among women than men - the excess being largest in middle-age, due to breast and cervical cancer. At older ages, the gap narrowed because of an increasing prevalence of prostate cancer. 51% of men and 25% of women aged 35-54 smoked cigarettes, while 29% of men and 41% of women aged 35-54 were obese (i.e. BMI > or =30 kg/m2). The prevalence of treated hypertension or measured blood pressure > or =140/90 mmHg increased about 50% more steeply with age among women than men, to 66% of women and 58% of men aged 65-74. Physical inactivity was highly prevalent but daily alcohol drinking was relatively uncommon. CONCLUSION: Diabetes, obesity and tobacco smoking are highly prevalent among adults living in Mexico City. Long-term follow-up of this and other cohorts will establish the relevance of such factors to the major causes of death and disability in Mexico.