HERMEX is a population-based study which try to evaluate the relative weight of cardiovascular risk factors in the population of Extremadura. This report provides the data obtained about microalbuminuria in a large Spanish population. Observational, cross-sectional, population-based study. A total of 3402 subjects were randomly selected from the Health Care System of Extremadura. The final sample included 2813 subjects (mean age 51·2 years, 53·5% female). Urinary albumin excretion rate (UAER) in first morning urine sample was analysed. Microalbuminuria was diagnosed when UAER was ≥ 22 in men or ≥ 31 mg/g in women. Prevalence of abnormal UAER in general population was 5·5% (microalbuminuria: 4·7%; proteinuria 0·8%). Microalbuminuria grew slightly in patients between 65 and 74 years and showed a dramatic increase in subject older than 75 years (P < 0·001). Men showed a high prevalence of microalbuminuria (5·8% vs. women 3·6%; P = 0·006 chi-squared test). Increased UAER was more common in obese subjects (6·7% vs. 2·3%, P < 0·001), hypertensive patients (8·3% vs. 2·3%, P < 0·001) and diabetic ones (10·9% vs. 3·7%, P < 0·001). The multivariate analysis showed a positive correlation of abnormal UAER with body mass index (BMI), systolic blood pressure, plasma creatinine and triglyceride levels. A low frequency of abnormal UAER was detected in a randomly selected sample of Spanish general population. This finding agreed with the low rates of cardiovascular mortality and morbidity observed in Spain in spite of a high prevalence of classic cardiovascular risk factors. © 2012 The Authors. European Journal of Clinical Investigation © 2012 Stichting European Society for Clinical Investigation Journal Foundation.