Those afflicted with bipolar disorder often suffer from substantial functional impairment both when in episode and when in remission. This study examined the psychometric properties of a brief assessment of psychosocial functioning, the Range of Impaired Functioning Tool (LIFE-RIFT), among subjects with bipolar I disorder. The study sample consisted of 163 subjects who presented with bipolar I disorder at intake into the NIMH Collaborative Depression Study (CDS). All LIFE-RIFT items come from the Longitudinal Interval Follow-up Evaluation (LIFE). Follow-up data that were used to examine the reliability and validity of the scale come from assessments of psychosocial functioning that were conducted 6, 12, 18, and 24 months after intake into the CDS. The results of factor analyses indicate that the scale items are measures of one construct, psychosocial functioning. The interrater agreement on the scale score was very good with an intraclass correlation coefficient was 0.94. The internal consistency reliability among the scale items was uniformly satisfactory over the four assessment periods, with coefficient alpha ranging from 0.78 to 0.84. Mixed-effect regression analyses showed that during mood episodes subjects were significantly more impaired than those in recovery. In conclusion, the psychometric properties of the LIFE-RIFT were examined in subjects with bipolar I disorder. The analyses from this longitudinal, observational study provide empirical support for the reliability and validity of the scale. The LIFE-RIFT provides a brief, inexpensive alternative to scales currently used to assess psychosocial functioning and can be easily added to semistructured assessments that are used in clinical and treatment outcome studies.