The purpose of this study was to reveal the clinical validity of the Japanese version of the Functional Assessment of Cancer Therapy-Anemia scale (FACT-An) in relation to hemoglobin level. We also analyzed patients' scores for the related FACT-General scale (FACT-G), the FACT Anemia subscale, and the FACT Trial Outcome Index-Anemia scale (FACT TOI-An) to determine which was the most sensitive to anemia measurements. Throughout Japan, we recruited 227 patients (mean+/-SD, 59+/-12.1 years old) diagnosed with a variety of cancers. We correlated the severity of anemia, as measured by hemoglobin levels, to scores on the FACT-An and on the other scales at baseline and at 3 months. The questionnaire completion rate was more than 98% at both time points. The FACT-An had high internal consistency (Cronbach's alpha coefficient >0.8). FACT-An scores were significantly and positively correlated with hemoglobin levels both at baseline (r=0.24; 95% CI=0.12 to 0.36; n=225) and at 3 months (r=0.24; 95% CI=0.10 to 0.36; n=204). FACT-G, FACT Anemia subscale, and FACT TOI-An scores also successfully discriminated between patients with lower (Hb <11.0 g/dl) and higher (Hb > or =11.0 g/dl) hemoglobin levels. Moreover, the changes of these FACT scores over 3-months could discriminate changes in hemoglobin level. The Japanese version of the FACT-An has higher clinical validity and can be used to appropriately assess health-related quality of life among Japanese cancer patients with anemia.