Girls of age 10-13 yr with Tanner stage I-III maturation status (n = 155) were measured using the Prodigy (GE Lunar) densitometer. Bone area (BA), bone mineral content (BMC), and bone mineral density (BMD) were assessed for the whole body, lumbar spine, and proximal femur using the Thin (T) and Standard (S) scan modes at years 1 and 3 of the study. The differences obtained between the T and S mode at year 1 were 1-2% for the lumbar spine and proximal femur and 5-11% for the whole body. For those girls whose default mode changed from T at year 1 to S mode at year 3, the estimated gain in BA, BMC, and BMD was 3.4%, 7.6%, and 3.1% respectively, lower than that obtained when scanning with the T mode at both times for the whole body. Small changes in magnitude but large intersubject variability were noted in BA, BMC, and BMD of the lumbar spine and proximal femur when scanned with the default mode of T at year 1 and S at year 3 compared to T or S at both years. Errors of this size are comparable to the changes expected with longitudinal intervention studies and are, therefore, clinically relevant.