This report describes the psychometric properties of the six-item Brief Instrumental Functioning Scale (BIFS), a self-report questionnaire for measuring instrumental functioning in community populations, and the results of a study to validate the scale among homeless persons. The participants in the study were 1,509 homeless persons, of whom 1,077, or 71 percent, were seriously mentally ill, substance dependent, or both. Confirmatory factor analysis provided evidence for the unidimensionality of the items in the scale. The BIFS had high internal consistency, respectable stability, and reasonable convergent validity. The BIFS is useful for community populations when self-report data on functioning are required. Additional research is needed to develop and validate comparable scales for assessing other domains of functioning.