Although there is a high prevalence of anxiety and depression amongst people receiving antiretroviral treatment (ART), many patients are not screened, diagnosed or referred for mental health problems. This study aims to determine whether public sector health care workers in South Africa observe, screen, diagnose and refer ART patients that show symptoms of common mental disorders. It also aims to ascertain the extent of mental health training received by public sector health care workers working in ART. The study was cross-sectional in design. Self-administered questionnaires were completed by 40 nurses and structured interviews were conducted with 23 lay workers across the five districts in the Free State between July 2009 and October 2009. STATA version 12 was used to perform statistical data analysis. The health care workers reported observing a high frequency of symptoms of common mental disorders among public sector ART patients. While 70% of nurses screened and diagnosed, only 40% of lay workers screened and diagnosed patients on ART for a mental disorder. Health care workers who had received training in mental health were more likely to screen or diagnose a mental disorder, but only 14% of the workers had received such training. We recommend that health care workers should receive task-specific training to screen and/or diagnose patients on ART for common mental disorders using the guidelines of the South African HIV Clinicians Society. A positive diagnosis should be referred to a health care practitioner for appropriate evidence-based treatment in the form of medication or psychotherapy.