A significant relation between religion and better health has been demonstrated in a variety of healthy and patient populations. In the past several years, there has been a focus on the role of spirituality, as distinctfrom religion, in health promotion and coping with illness. Despite the growing interest, there remains a dearth of well-validated, psychometrically sound instruments to measure aspects of spirituality. In this article we report on the development and testing of a measure of spiritual well-being, the Functional Assessment of Chronic Illness Therapy-Spiritual Well-Being (FACIT-Sp), within two samples of cancer patients. The instrument comprises two subscales--one measuring a sense of meaning and peace and the other assessing the role offaith in illness. A total score for spiritual well-being is also produced. Study 1 demonstrates good internal consistency reliability and a significant relation with quality of life in a large, multiethnic sample. Study 2 examines convergent validity with 5 other measures of religion and spirituality in a sample of individuals with mixed early stage and metastatic cancer diagnoses. Results of the two studies demonstrate that the FACIT-Sp is a psychometrically sound measure of spiritual well-being for people with cancer and other chronic illnesses.