Background: Social class is an important aspect of all societies. Social class differences in the prevalence of common mental disorder are likely to vary according to time, culture and stage of economic development. The present study aimed to investigate the intermediaries between social class and common mental disorder in a deprived area of Northeast Brazil. The Self Reporting Questionnaire (SRQ) was used to estimate the prevalence of common mental disorder. Method: A cross-sectional survey was conducted of 683 adults, aged 15 years and over, living in a random sample of private households in area II of Olinda, Recife Metropolitan Region, Pernambuco, Brazil. Results: Poor education (odds ratio, OR 2.5, 95% CI 1.2–5.2; <5 years vs ≥11 years education) and low income (OR 2.4, 95% CI 1.0–5.6; ≤1/4 minimum wage vs >1 minimum wage) were independently associated with the prevalence of common mental disorder. Univariate associations with occupation, housing conditions and possession of household appliances could be understood in part by their association with poor education. Conclusions: Brazil has the second most unequal distribution of income in the world. Disparity in educational attainment could be one of the most important factors perpetuating social inequalities in psychiatric disorder in the country.