Very few studies described the prevalence of mental disorders in the workplace by using standard diagnostic criteria. A two-stage case-control study of anxiety and depression was initiated by Electricité de France and Gaz de France, the French Nationwide Company producing electricity and gas, using the General Health Questionnaire with 12 items as a screening test and the Composite International Diagnostic Interview as diagnostic instrument. Its aim was to point out occupational situations that promote the occurrence of anxiety and depression crises. Annual prevalence of depression was estimated at 7.6% in men (95% confidence interval [CI], 5.5 to 9.7) and at 17.9% in women (95% CI, 9.9 to 25.8), and that of anxiety at 9.6% (95% CI, 6.9 to 12.3) in men and 26.3% (95% CI, 17.2 to 35.4) in women. Workers in hazardous occupations were found to be protected from these disorders, whereas supervisory staff tended to be prone to developing them. Important changes in work or in its organization seemed to be risk factors. Extra-professional variables and occupational characteristics were included in a logistic regression model. The odds ratios corresponding to recent job changes and a supervisory position were significantly elevated (odds ratios = 1.7 and 2.4, respectively). These results suggest that some occupational events, such as major changes in work content or organization may cause or precipitate anxiety and/or depressive disorders.