In order to study the features of sinonasal polyposis (SNP) on CT, 100 consecutive coronal sinus CT examinations done for chronic inflammatory sinonasal disease were reviewed. The CT findings of the 27 fully documented SNPs were analyzed. All our SNPs were bilateral. There was a strong tendency for extensive involvement. Nasal polyps were seen in 22 of 27(81%); bony trabecular deossification in 23 of 27 (85%); widening of infundibulum in 26 of 27 (96%). We discovered a new sign "truncation of the bony middle turbinate", where the bulbous part of bony middle turbinate was missing, in 15 of 26 (58%) of SNP patients without a previous history of middle turbinectomy, 12 of 15(80%) were bilateral. The one SNP patient (1 of 27) with previous middle turbinectomy was not regarded to be real truncation. Truncation of the bony middle turbinate is a characteristic and easily recognizable ancillary sign, and is not seen in other patterns of sinusitis. Together with other features on coronal sinus CT, this adds diagnostic confidence in diagnosing sinonasal polyposis.