The Pittsburgh Sleep Quality Index (PSQI) measures sleep quality and disturbance retrospectively over a 1-month period using self-reports. Although the PSQI has been used in a variety of populations, published psychometric data are limited. The goal of this study was to examine psychometric properties of the PSQI among four populations: bone marrow transplant patients (n = 155); renal transplant patients (n = 56); women with breast cancer (n = 102); and women with benign breast problems (n = 159). Results supported PSQI internal consistency reliability and construct validity. Cronbach's alphas were 0.80 across groups and correlations between global and component scores were moderate to high. PSQI scores were moderately to highly correlated with measures of sleep quality and sleep problems, and poorly correlated with unrelated constructs. Individuals with sleep problems, poor sleep quality, and sleep restlessness had significantly higher PSQI scores in comparison to individuals without such problems.