The infrastructure to deploy security standards is often lacking during modern telemedicine. As a branch of watermarking technique, the fragile watermarking provides us a good solution to the authentication for telemedicine. A fragile watermarking algorithm based on least significant bit (LSB), hash function and chaotic sequence is presented in this paper. To resist counterfeiting attack, the watermark signal is extracted from the original image, so different image has different watermark signal. The watermark signal is then processed by exclusive-OR (XOR) operation with the chaotic sequence in order to increase the security of the watermarking algorithm. Finally, it is embedded in the LSB of the host image. The experimental results show that the watermark is invisible; the watermark algorithm has good ability to resist counterfeiting attacks. Furthermore, after the watermark is extracted from the watermarked image, it is exactly the same as the original one. This kind of watermarking algorithm can e used for real-time diagnosis.