The Alberta Infant Motor Scale (AIMS), an observational assessment scale, was constructed to measure gross motor maturation in infants from birth through independent walking. Based upon the literature, 58 items were generated and organized into four positions: prone, supine, sitting and standing. Each item describes three aspects of motor performance--weight-bearing, posture and antigravity movements. Content validation of the instrument was accomplished through a mail survey of Canadian pediatric physical therapists and consultation with an international panel of experts. Five hundred and six infants, age-stratified from birth through 18 months, participated in the reliability and validity testing of the AIMS. In addition, 20 infants who were experiencing abnormal motor development and 50 infants at risk for motor disorders were assessed and compared with the results of the full-term sample. Results to be presented include: 1) test-retest and inter-rater reliability estimates; 2) correlations between the AIMS and the Bayley and Peabody motor scores; and 3) scaling of the items along the age continuum for normal motor development.