If cancer survival is reported to be worsening over time or inferior compared to other countries, politicians and health-care workers may get blamed because suboptimal care is presumed to be the cause. Yet, a variety of reasons exist for cancer survival statistics to change for the worse, of which deterioration of care is only one. Another explanation is that the improved diagnosis of premalignant lesions causes survival statistics to reflect only the most aggressive cancers-those with the poorest prognosis. In addition, deleterious changes in the distribution of prognostic factors and in the distribution of sociodemographic characteristics may negatively affect survival proportions. In this article, we identify the pitfalls that might be encountered in comparisons of published, population-based survival data from different time periods or populations.