The advantages of customized Laboratory Information Management's Systems (LIMS) are their focus on the special aspects of their users' needs. Differences in the research and development or production chain in the individual organizations lead to an increase of interest in customized systems. Usually, also for customized systems, the core software is commercially available. The individual application modules as the Customized part of the LIMS are the most critical elements within the validation process. The topic of this paper is to give an example of the validation of a customized analytical LIMS. Validation of complex computerized systems guarantees the intended use and is therefore an unavoidable requirement of authorities. The audit of the supplier of the individual programmed modules, the user requirement specifications and the acceptance testing and results, respectively, on the software are of special interest within a customized LIMS. The hardware suitability and the principal processing routines are also a very important part of the whole validation process, but they will not be discussed in detail in this paper.