The setting for this retrospective cohort study was a specialised tuberculosis unit in Madrid, Spain. The objective was to describe the risk factors for multidrug-resistant tuberculosis (MDR-TB). The medical records of all patients admitted to the unit were reviewed retrospectively to identify factors associated with multidrug resistance. Patients with positive culture for M. tuberculosis and with available drug-susceptibility tests were included. The variables assessed were age, gender, country of origin, homelessness, alcohol consumption, intravenous drug use, methadone substitution therapy, contact with a tuberculosis patient, sputum smear, site of disease, previous tuberculosis treatment, HIV infection, history of imprisonment, diabetes mellitus and chronic obstructive pulmonary disease. Thirty patients with MDR-TB and 666 patients with non-MDR-TB were included from the years 1997 to 2006. The only factors associated with MDR-TB in multivariate analysis were previous tuberculosis treatment (OR: 3.44; 95% CI: 1.58-7.50; p = 0.003), age group 45-64 years (OR: 3.24; 95% CI: 1.34-7.81; p = 0.009) and alcohol abuse (OR: 0.12; 95% CI: 0.03 to 0.55; p = 0.003). In our study, patients who had had previous treatment for tuberculosis, who were 45-64 years of age or who had no history of alcohol abuse were more likely to have MDR-TB.