UDDI registries are today the standard way of publishing information on Web services. They can be thought of as a structured repository of information that can be queried by clients to find the Web services that better fit their needs. Even if, at the beginning, UDDI has been mainly conceived as a public registry without specific facilities for security, today security issues are becoming more and more crucial, due to the fact that data published in UDDI registries may be highly strategic and sensitive. In this paper, we focus on authenticity issues by proposing a method based on Merkle Hash Trees, which does not require the party managing the UDDI to be trusted with authenticity. In the paper, besides giving all the details of the proposed solution, we show its benefit with standard digital signature techniques.