Background/Aims: Two different methods of colon transit time studies (4-day method and 7-day method) are commonly used in Korea. The aim of this study was to define the normal values and to compare the clinical usefulness of the two methods. Methods: Seventy-five healthy volunteers and 101 patients with chronic constipation were enrolled in this study. Radio-opaque markers were given for 3 days and simple abdominal radiographs were obtained on the 4th and 7th days. Colon transit times estimated by only 4th-day film (CTT-4) and by both 4th and 7th-day films (CTT-7) were compared. Results: The upper limits of normal volunteers without constipation were 56 hours in CTT-4 and 61 hours in CTT-7. In selecting constipated patients with delayed transit, the results of both methods were comparable (kappa=0.823, p<0.01). However, the 4-day method could not differentiate patients with severely delayed transit from patients with mildly delayed transit. Conclusions: Four-day method is useful in selecting constipated patients with delayed transit but it does not give relevant information on the severity of delayed transit. Seven-day method can be used in selecting patients with severely delayed transit. (Korean J Gastroenterol 2001;38:241-246)