BACKGROUND: Previous studies have suggested that interviewer-administered questionnaires can under-estimate the prevalence of depression and suicidal ideation when compared with self-administered ones. We report here on differences in prevalence of reporting mental health between four questionnaire delivery modes (QDM). METHODS: Mental health was assessed using the Shona Symptom Questionnaire (SSQ), a locally validated 14-item indigenous measure for common mental affective disorders. A representative sample of 1495 rural Zimbabwean adolescents (median age 18) was randomly allocated to one of four questionnaire delivery modes: self-administered questionnaire (SAQ), SAQ with audio (AASI), interviewer-administered questionnaire (IAQ), and audio computer-assisted survey instrument (ACASI). RESULTS: Prevalence of common affective disorders varied between QDM (52.3%, 48.6%, 41.5%, and 63.6% for SAQ, AASI, IAQ, and ACASI respectively (P<0.001)). Fewer participants failed to complete SSQ using IAQ and ACASI than other methods (1.6% vs. 12.3%; P<0.001). Qualitative data suggested that respondents found it difficult answering questions honestly in front of an interviewer. LIMITATIONS: Direction of accuracy cannot be ascertained due to lack of objective or clinical assessments of affective disorders. CONCLUSIONS: Estimates of prevalence of psychosomatic symptoms and suicidal ideation varied according to mode of interview. As each mode's direction of accuracy remains unresolved evaluations of interventions continue to be hampered.