Several epidemiological studies have shown an association between the season in which certain cancers are diagnosed and survival, with diagnosis in summer and autumn being associated with better survival. In this study, we have added resolution to the analysis of seasonality in cancer survival by considering mortality within several nonoverlapping time periods following diagnosis, thereby quantifying the separate contributions of mechanisms operating in the short term and in the longer term. We found evidence of seasonality acting on mortality within 2 distinct periods following diagnosis. Diagnosis in the summer was associated with substantially decreased mortality within the first month of diagnosis compared with winter in men with prostate cancer, those of both sexes with colorectal or lung cancer, and most strikingly, amongst women with breast cancer (hazard ratio 0.81 [95% confidence interval 0.75-0.86]). Adjusting for monthly variations in general mortality greatly attenuated the seasonal effects on short-term mortality. At long-term follow-up (>5 years), there was a consistent shift in the seasonality pattern, with autumn diagnosis alone being associated with decreased mortality, both in female breast cancer cases and in lung cancer cases of both sexes. We conclude that the higher survival observed amongst patients diagnosed in summer and autumn is predominantly a short-term phenomenon that is largely attributable to generally higher mortality in winter. However, the distinct mortality reduction observed in the long term amongst those diagnosed in the autumn, especially amongst breast cancer patients, may indicate the presence of a seasonally variable protective mechanism.