Numerous medical on-line services have already been established in the world-wide internet. In connection with the Information service TELESCAN, sponsored by the EU, a pilot project has been initiated which offers a radiological "second opinion" via the transmission of radiological findings and images that have been previously rendered anonymous. In addition to a description of the basic implementation, tests of the diagnostic certainty of the transmitted cranial computed tomographs have been performed. The CT images were digitized with a document camera, transmitted over the Internet, and then evaluated on the receiver's monitor. Both the transfer of originally generated digital image files (in ACR-NEMA or DICOM) as well as graphic files after digitization of X-ray films, for example by a document camera, is possible via electronic post (e-mail). Visualization by the receiver requires the use of current proprietary software for special medical image formats, while standard graphic formats such as GIFF or JPEG can be visualized with the usual Internet software. In an ROC analysis, 56 individual images of cranial computed tomographs, half with pathological findings such as space-occupying lesions, infarcts, or brain edema, were tested with regard to the diagnostic certainty after digitization and transmission. In comparison with the original film findings, there was a slight but statistically not significant reduction in diagnostic certainty of the images evaluated on screen after transfer via the Internet. We believe that this result is due to the low local resolution, low dynamic range, the high image noise and of CT arising from the window technique. The same parameters are probably valid for MRI. The result cannot be applied to conventional radiography including mammography because, in comparison to the mentioned image techniques, their local resolution is high and image noise is considerably lower.