The main purpose of the study was to evaluate quality of life (QOL) among cancer patients using the WHOQOL-100 instrument and to see if any significant differences were seen in cancer stages, treatment status and prognosis. This study consisted of two parts; qualitative and quantitative. For the qualitative study, two focus groups were conducted by medical professionals to establish the applicability of the WHOQOL instrument in evaluating the QOL of cancer patients, but most participants were negative about using a generic instrument such as WHOQOL. For the quantitative study, 197 cancer patients (average age 55.86) from eight medical centers using the WHOQOL instrument, in addition to each patient's information sheet filled in by their own physicians, were analyzed. The average overall QOL score was 3.39. There was high reliability (Cronbach's alpha = 0.9685) and a high correlation between the psychological and the environmental domains (r = 0.7021), the physical domain and the level of independence (r = 0.6031) and social relations and the environment (r = 0.6856) and between health conditions perceived by patients and QOL scores. In addition, differences by gender, treatments and cancer sites were also found to be significantly different at the 5% significance level. The results indicated that the WHOQOL core instrument was sensitive enough to evaluate the QOL of cancer patients.