There is no one established approach to the measurement of spiritual quality of life (QOL). Available instruments are based on various theoretical components. We used a multi-instrument approach to measure the spiritual domain of QOL that adds to our understanding of a participant self-definition of spiritual QOL. In total, 103 participants with advanced cancer receiving radiation therapy were enrolled in this study. Most were Caucasian, male, and had advanced lung, head and neck, or gastrointestinal cancer. Two instruments, the Spiritual Well-Being Linear Analogue Self Assessment (SWB LASA) and the 12-item Functional Assessment of Chronic Illness Therapy-Spiritual (FACIT-Sp-12), were used to measure spiritual QOL at enrollment and 4, 8, and 27 weeks after enrollment. Analyses included descriptive statistics, Spearman correlations, stepwise multiple regression, and repeated measures analysis of variance. There was a strong association between SWB LASA and FACIT-Sp-12 total scores. However, FACIT-Sp-12 items defining SWB LASA scores varied over time. Two to three of the FACIT-Sp-12 items explained approximately two thirds of the variance in the SWB LASA scores at each time point with the exception of 4 weeks after enrollment. SWB scores were strongly associated with all QOL domains. In research and clinical care, SWB must be treated as a complex concept that has the potential to change over time. Although a single-item measure of SWB provides valuable information and is strongly associated with the multiple item FACIT-Sp-12, our more detailed inquiry using the FACIT-Sp-12 provides additional guidance for the design and timing of spiritual support interventions.