BACKGROUND A direct quantitative measurement of locomotor activity in an individual's own environment over an extended period may help in evaluating the impact of impairments in neurological disorders. OBJECTIVE To investigate the reliability and validity of activity monitoring in neurological patients and healthy subjects. METHODS Initial reliability studies were completed on 10 healthy subjects and 10 mobility restricted neurological patients. Validity was investigated using 7 days of ambulatory monitoring with the Step Watch( step activity monitor, laboratory based measures of gait and the Rivermead Mobility Index (RMI) in 10 patients with multiple sclerosis, 10 with Parkinson's disease, and 10 with a primary muscle disorder. Additionally, 30 healthy subjects participated in the study. Two clinical illustrations of ambulatory monitoring are provided. RESULTS The mean (range) right step count of 7 days of monitoring in both healthy and neurological patients proved a reliable measure of activity (intra-class correlations 0.89 and 0.86 respectively). The 7 day mean (range) right step count was 5951 (2886-9955) in healthy subjects, 3818 (1611-5391) in patients with Parkinson's disease, 3003 (716-5302) in those with muscular disorders, and 2985 (689-5340) in those with multiple sclerosis. A moderate correlation was noted between 7 day mean step count and gait speed (r = 0.45, p = 0.01) in the grouped neurological patients but not the RMI (r(s) = 0.3, p = 0.11). CONCLUSION Ambulatory monitoring provides a reliable and valid measure of activity levels. Neurological patients, living independently, demonstrate lower activity levels than healthy matched controls. Ambulatory monitoring as an outcome measure has potential for improving the evaluation of ambulation and providing insight into participation.