This study aims at developing a single numerical measure that represents a depressed patient's individual burden of illness. An exploratory study examined depressed outpatients (n = 317) followed by a hypothesis confirmatory study using the NIMH STAR*D trial (n = 2,967). Eigenvalues/eigenvectors were obtained from the Principal Component Analyses of patient-reported measures of symptom severity, functioning, and quality of life. The study shows that a single principal component labeled as the Individual Burden of Illness Index for Depression (IBI-D) accounts for the vast majority of the variance contained in these three measures providing a numerical z score for clinicians and investigators to determine an individual's burden of illness, relative to other depressed patients.