OBJECTIVES: The aims of this study were to evaluate the test–retest reliability and the validity of self-reported duration of computer use at work. METHODS: Test–retest reliability was studied among 81 employees of a research department of a university medical center. The employees filled out a web-based questionnaire twice with an in-between period of 14 days. Validity was studied among a group of 572 office workers who participated in an epidemiologic field study. A software program recorded the duration of computer use at work during the 3 months preceding the questionnaire. RESULTS: The percentages of agreement for test–retest reliability were 75% [95% confidence interval (95% CI) 64–84] for total computer use and 67% (95% CI 55–77) for mouse use. The percentages of agreement between self-report and registration were 18% (95% CI 15–21) for total computer use and 16% (95% CI 13–19) for mouse use. Misclassification was mainly nondifferential in nature, since all of the evaluated subgroups showed at least 75% misclassification. CONCLUSIONS: The use of self-reports lead to the misclassification of exposure to computer use for more than 80% of all persons. This misclassification is predominantly nondifferential in nature and can only partly be explained by limited test–retest reliability.