We present a brief measure of caregiver burden, the Mood Disorder Burden Index (MDBI), for use with family members and close friends of adults with major depressive disorder (MDD) or bipolar disorder (BD). The MDBI assesses burden in three core domains (patients' mood symptoms, caregivers' worry about the future, and caregivers' interpersonal difficulties with the patient) and includes an optional module that assesses caregiver burden associated with patients' pharmacotherapy or psychotherapy. The MDBI was administered to caregivers of older individuals (i.e., 58 years and older) with MDD (n=123) or BD (n=38) who were receiving treatment through a research study. Analyses indicated evidence of convergent and discriminant validity of the new measure well as internal consistency within both caregiver groups. It will be important for future research to administer the MDBI to caregivers of middle-aged and older patients as well as those receiving treatment through inpatient settings or community outpatient clinics.