Medical treatment of cancer often entails a trade-off between outcomes of two different attributes: quality of life (QOL) and length of life (LL). This process of weighing advantages and disadvantages seems to be influenced by different factors. The main purposes of this study were (a) to investigate the relative importance of different factors on the trade-off and (b) to explore the relationship between these importance ratings and personal characteristics. We asked 199 patients with cancer to indicate to what degree they consider a number of factors to be of importance if they had to choose between two treatment modalities that differ in their expected outcomes concerning QOL and LL. The seven factors were their age at the time of decision, having a partner, having children, inability to work due to side-effects of the treatment, the nature of the side-effects, disease-related life expectancy and baseline QOL. The results indicate that six of the seven factors were of considerable to great importance when a treatment choice had to be made. The negative effects of treatment on the ability to work did not seem to be a very important consideration. Patient age and education were positively associated with importance ratings.