Objective: Evaluate psychometric properties of the Functional Arm Activity Behavioral Observation System (FAABOS) for measuring hemiparetic arm use. Participants and Measures: All participants acquired their brain injury > 1 year prior to study entry; most had mild-to-moderate upper-extremity hemiparesis. In Study 1, 9 stroke survivors wore accelerometers and were videotaped for 15 min in the hospital or at home after they were asked to behave as usual. In Study 2, 1 traumatic brain injury and 8 stroke survivors wore accelerometers and were videotaped at home for 3 days with a motion-triggered camera. Observers independently rated 15-min segments of the Study 1 and 2 videotapes in 2-s blocks with a 4-step arm-activity coding scheme. Results: Interrater reliability was excellent; the mean Cohen's κ in each study was ≥ .84. For data from both studies combined, validity was supported by a strong correlation between amount of hemiparetic arm functional activity, as determined by the observers, and the ratio of hemiparetic to other arm movement, as determined by accelerometry. Conclusion: FAABOS reliably and validly quantifies amount of spontaneous hemiparetic arm activity outside the laboratory. © 2009 American Psychological Association.