Whereas most previous studies have assessed optimism/pessimism as a unidimensional construct, there is increasing awareness that optimism and pessimism may represent two partially independent dimensions. In this study, the role of optimism and pessimism for the maintenance of psychological well-being was assessed in 161 newly diagnosed cancer patients. Before the start of chemotherapy, more positive affect balance was associated with higher optimism and lower pessimism. Over the course of 9 months following diagnosis, pessimism predicted negative change in affect balance, whereas no effect of optimism appeared. Higher levels of perceived side-effects of chemotherapy were associated with negative change in affect balance. However, an interaction effect of perceived side-effects with pessimism indicated that this was only the case in patients with above-median levels of pessimism. The conclusion is drawn that it may be more important to be less pessimistic than to be optimistic when diagnosed with cancer.