Background: The personality variables optimism and pessimism are potential risk factors for disorders commonly treated with antidepressants. Aims: To evaluate optimism and pessimism as predictors of initiating and ending an antidepressant treatment. Methods: Data consisted of 29,930 public sector employees with no record of diagnosed depression. Optimism and pessimism were measured using the Revised Life Orientation Test (LOT-R) at baseline. The data of purchases of antidepressants were from the national Drug Prescription Register. Results: During the mean follow-up of 4.4 years, 1681 participants initiated and of them 1288 ended an antidepressant treatment lasting at least 100 days. In the adjusted model, high optimism was associated with a lower likelihood of starting antidepressant medication treatment (hazard ratios, HR, 0.67, 95% CI 0.62-0.73) and a higher likelihood of stopping the treatment (HR = 1.18, 95% CI 1.08-1.30). High pessimism was associated with a higher likelihood of starting antidepressant medication treatment (HR = 1.27, 95% CI 1.16-1.38) and a lower likelihood of stopping it (HR = 0.89, 95% CI 0.80-0.98). These associations remained after optimism score was adjusted for pessimism and vice versa or those with symptoms of mental health problems at baseline were removed from the analyses. Conclusions: Low optimism and high pessimism are independently associated with an increased likelihood of initiating antidepressant medication treatment, but with a decreased likelihood of ending it during the follow-up.