A comparative study of quantitative hemidiaphragmatic ultrasound with fluoroscopy was undertaken in 30 patients referred for investigation of suspected hemidiaphragmatic movement abnormality. The aim of this study was to determine whether assessment with ultrasound or fluoroscopy differed, and which technique appeared more suitable in the investigation of hemidiaphragmatic movement disorder. There were four technical failures using fluoroscopy (13%), compared with none using ultrasound. Using the normal ranges of right to left ratio of maximal excursion (0.5-2.0 for fluoroscopy and 0.5-1.6 for ultrasound) there was concordance in 21 out of 26 (81%) patients. All cases of abnormality on fluoroscopy were seen on ultrasound. Four of the discordant cases had excursions on the lesser side in the normal range on ultrasound suggesting a milder movement abnormality detected by ultrasound than by fluoroscopy. Sniff testing conferred no advantage over quantitative testing. Ultrasound has technical, qualitative and quantitative advantages over fluoroscopy and should be the method of choice in the investigation of suspected hemidiaphragmatic movement abnormality.