For linkage analysis using pedigree data, it is important to eliminate contradictions concerning Mendelian inheritance from genotypic data. Such contradictions may derive from either genotyping errors or pedigree errors. We implemented an error-checking algorithm in a World Wide Web-based program that can be used through the Internet even by computer nonspecialists. This program, named Checkfam, uses two error-checking algorithms to detect and report contradictions concerning Mendelian inheritance. With this program, users can guess the causes of genotype errors (technical problems in genotyping, pedigree misrecording, or errors in input data concerning familial relationships). The present program will be useful to researchers for checking genotypic data and for preparing correct input files for linkage analysis.