Many existing models posit that cognitive processing style is an important factor affecting self-perceived positive changes. In this study, the effects of explanatory style (the manner in which people cognitively process and explain why they experience good and bad events) on both posttraumatic growth (PTG) and posttraumatic stress disorder (PTSD) symptoms were examined among 90 Chinese women with breast cancer. It was found that explanatory style for good events, but not for bad events, was significantly associated with self-reported PTG. Women who attributed the causes of positive events to internal, global and stable factors tended to report more posttraumatic growth. In contrast, explanatory style for bad events, as opposed to good events, was significantly and positively correlated with PTSD symptoms. Among the three dimensions of explanatory style (internal, stable and global), the tendency to globalise the causes of good and bad events were the most important predictors of self-reported PTG and PTSD symptoms, respectively. While enhancing an optimistic explanatory style for bad events might reduce posttraumatic stress symptoms, cultivating an optimistic explanatory style for good events is likely to increase self-perceived positive changes after breast cancer diagnosis and treatment.