Objective: To assess the prevalence of common mental disorder and its related factors in primary health care. Methods: Cross-sectional study with 607 individuals in a primary health care service. The instrument of the study was the Self Reporting Questionnaire 20. Results: Out of the interviewed subjects, 31.47% showed greater probability of occurrence of a common mental disorder. The following predictive variables were associated with a lower probability of occurrence of common mental disorder: sex, being single, being a student or a worker with signed labor, having higher education levels and income over four times the minimum wage. The variables associated with a higher probability of occurrence of a common mental disorder were being self-employed, housewife, with children, having lower education level and low income. Conclusion: The prevalence of a common mental disorder was high and the associated factors were: being female, divorced, Asian, aged between 18 and 59, housewife, with children, having four to seven years of education, income up to one minimum age and living in a borrowed or donated house.