OBJECTIVE To estimate the prevalence and risk factors of obesity in a sample of primarily urban Mexican Americans (the fastest growing segment of the US population), and to examine the association between obesity and co-morbid conditions. METHODS Subjects were participants from a cross-sectional, population-based prevalence study. Data were collected on 6038 noninstitutionalized self-identified Latinos of primarily Mexican American ancestry age > or = 40 years from six census tracts in Los Angeles County, California. Obesity was defined having a body mass index > or = 30.0 kg/m2. RESULTS The overall prevalence of obesity was 50% (females 54% vs males 43%, P < .0001). Stepwise logistic regression analyses revealed that obesity was positively associated with being a former smoker (OR 1.5, P = .0009), being unemployed (OR 1.5, P < .0001), and with female sex (OR 1.3, P = .02). Obesity was negatively associated with being single or divorced (OR .8, P = .014), being a current smoker (OR .6, P < .0001), and with age > or = 70 years (OR .4, P < .0001). After adjusting for sex and age, obesity was significantly associated with the following systemic comorbidities: hypertension, heart failure, arthritis, diabetes, angina, back pain, and asthma (P < .01). CONCLUSION Because of the high prevalence of obesity and its associated systemic co-morbidities, an evaluation of current intervention programs is needed to determine the most effective approach to help decrease the prevalence of obesity and the risk of associated co-morbidities in this the fastest growing segment of the US population.