To measure patients' QOL in the daily practice of clinical oncology, we developed and tested the Care Notebook. This instrument has 24 questions expressed in single words or short phrases to make it more acceptable to patients. The Care Notebook, EORTC QLQ-C30 and FACIT-Sp-12 were administered to 249 outpatients with cancer. Construct validity was investigated by cluster analysis and multitrait scaling analysis. The results showed that three scales (physical well-being, mental well-being, and life well-being) could explain 55% of the variance in scores. The life well-being scale could be divided into subscales of Daily Functioning, Social Functioning, and Subjective QOL. Multitrait scaling analysis confirmed convergent and discriminant validity of these scales and subscales. Internal consistency and test-retest reliability were favorable. Differences in Care Notebook scores were also consistent with differences in performance status rating (known-groups validity), and Care Notebook scores correlated with EORTC QLQ-C30 and FACT-Sp-12 scores (concurrent validity). The Care Notebook allows clinical oncologists to easily collect valid and reliable QOL information of physical, mental, and life well-being repeatedly and with minimal burden on patients.