A simple device to measure the rib hump deformity in scoliosis is described. The importance of defining the plane of measurement rigidly was stressed. The visible rib humps from 30 scoliotic patients were measured in the form of plotted curves by a group of paramedical and medical staffs. Highly reproducible results were obtained. The expected difference between two curves measured on a random patient by a single investigator was .116 cm, and the expected difference between the curves made by two random investigators on the same patient was .167 cm. This instrument is useful in charting the progress of the rib hump deformity in daily clinical practice.