Previous studies on pessimism and optimism in organizations have mainly focused on productivity and performance. The aim of the current study was to investigate the role of optimism in the relationship between stress at work and health. An Internet-based questionnaire was sent to 1345 female and male employees at both managerial and non-managerial level working in a Swedish telecom company. Altogether 950 employees (71%) participated in the study. The dimensions of work stress examined were quantitative demands, job insecurity, work and family conflict and lack of control. The results of the regression analyses showed that a low level of optimism had a direct effect on health problems. Optimism moderated the relationship between work insecurity and health symptoms. However, optimism did not act as a moderator between the quantitative demands, work and family conflict, lack of control and health. Future research about work stress and health should address personality factors such as optimism.