While quality of life (QOL) assessment is becoming more common, interpreting the results remains problematic. This paper demonstrates an approach to developing clinically-based interpretations for QOL outcomes, using the QLQ-C30 as an example. The results from 14 published QLQ-C30 studies which group patients by performance status, weight loss, toxicity, extent or severity of disease are collated. Groups with lower clinical status generally have worse QOL. The largest differences are between performance status groups, and the smallest differences are between groups of patients with local disease and those with metastases. The physical and role scores have the largest ranges of means across patient groups, and show the largest differences between clinical groups, while the cognitive and emotional scores have the smallest ranges of means and differences. Sicker groups have larger score standard deviations than healthier groups. Relatively large and small means and differences, and corresponding effect sizes, are presented. Collectively, the results provide a sense of the relative sizes of means and of differences, and of the types of clinical groups which give rise to them, thereby providing clinically-based benchmarks by which to interpret QLQ-C30 results.