The psychological impact of a cancer diagnosis can extend through treatment, well into cancer survivorship and can be influenced by a range of psychosocial resources. At different stages in this trajectory, optimism is known to affect well-being directly. This study focusing upon the potential to flourish after cancer, investigates the relationship between optimism and positive affect during cancer survivorship together with four possible mediators: social support, fighting spirit, internal health locus of control and cancer worry, all of which have been shown to be important predictors of well-being in cancer patients. Participants (n = 102) from online cancer forums completed standardized questionnaires, and path analysis confirmed that optimism had a direct effect on positive affect in cancer survivors. Social support and fighting spirit were also shown to be significant mediators of this relationship, accounting collectively for 50% of the variance in positive affect. Whilst cancer worry and internal health locus of control could be predicted from levels of optimism, they did not mediate the optimism–positive affect relationship. Efforts to promote optimism and thus encourage fighting spirit at diagnosis through treatment may be worthwhile interventions, as would ensuring appropriate social support through the trajectory. Copyright © 2012 John Wiley & Sons, Ltd.