[en] To clarify the cause of incurvated nail, the relation between the nail plate and the distal phalanx of the patient of incurvated/ingrown nail and normal subjects was compared, by measuring CT scan images. The investigation was made on 12 incurvated nails, 8 ingrown nails and 8 normal nails of the first toes which were clinically diagnosed as such. Each toe was scanned at 1 mm intervals with the same CT scanner under the same conditions. The pictures taken on the film were scanned and input into a computer. The transverse width of the distal phalanx and the width of the nail plate were measured on each slice. The minimum transverse width of the distal phalanx in both stages of slightly and intensively incurvated nail was smaller as compared to that in the stages of normal nail and ingrown nail. The ratios of the width of the nail plate to the minimum transverse width of the distal phalanx in the stages of normal nail, ingrown nail and intensively incurvated nail were nearly equal, while the ratio in the stage of slightly incurvated nail was outstandingly large. Based on the results of this investigation, it can be considered that the transverse width of the distal phalanx is reduced first, and consequently, the incurvation of the nail plate occurs, then the width of the nail plate is reduced as the incurvation becomes more intense. (author)