Quality-of-life instruments have provided important advances in measuring the quality of life of pediatric patients receiving treatment for cancer. However, the bases of these instruments have not included first-hand reports from the patients; thus, these instruments may be conceptually incomplete. We directly solicited from pediatric patients their perspectives regarding their quality of life during treatment for cancer. We conducted two pilot studies: 23 patients (aged 8-15 years) participated in the first, a cross-sectional study; and 13 patients (aged 10-18 years) participated in the second, a 2-year longitudinal study. Data were analyzed by using a semantic-content method, and the following six domains were recognized in data from both of the studies: symptoms, usual activities, social/family interactions, health status, mood, and the meaning of being ill. These domains were compared with those of seven established pediatric oncology quality-of-life instruments, none of which included all six of these domains; the domain most frequently missing was the meaning of being ill domain. Here we present a new definition of the quality of life of pediatric oncology patients that is based on six domains; this definition may ensure the completeness and sensitivity of these important instruments.