[en] The 1998 nationwide survey of the exposure of the Swiss population by medical X-rays indicated that 5.4 million X-ray examinations (dental excluded) were performed annually in Switzerland (762 per 1000 population), leading to an annual collective dose to the population of 7100 Sv and an average annual effective dose of 1.0 mSv/caput. The contribution of computed tomography (CT) was found to be 6% of the total in terms of frequency and 28% in terms of the collective dose. In the course of 2004 an updating study indicated a 70% increase of CT examinations, which, combined to a 20% increase in the average effective dose per CT procedure, lead to an increase by a factor of 2 in the CT contribution to the collective dose due to medical X-rays. This significant increase cannot be explained by a higher number of CT scanners, since the number of units in Switzerland did not show a big change over this period of time (187 in 1998 and 200 in 2004 - 7% increase). The present work aimed at assessing the trend of CT in Switzerland and to compare it to that of fluoroscopy (FL), magnetic resonance (MR) and ultrasounds (US). The study covered four university hospitals and the frequency data concerning the various radiological modalities was collected over several years (9 to 14 years, depending on the hospital). The frequency data was either extracted from the hospital's annual report or provided by the radiology department. The investigation showed a significant increase in the number of CT procedures, a factor 2 on average over an 8-year period (1997-2005). This increase is accompanied by a reduction of non vascular fluoroscopy procedures, a more or less constant number of angiographies, interventional procedures, and US examinations, and an increase of MR procedures (76% on average) in the same period of time. (author)