A rule-based expert system called FABHELP has been developed that applies the recently proposed French-American-British (FAB) criteria for diagnosing acute myeloid leukemia and myelodysplastic syndromes. The program diagnoses Ml-M7 and myelodysplastic syndromes given the percentages of the cell types. It was developed in Arity Prolog Version 5.1 and runs on IBM PCs. FABHELP asks the user for the percentage of cell types and requests specific information concerning the quality of the specimen and ancillary tests such as nonspecific esterase staining. The diagnosis is printed on the screen along with the rule used to make the diagnosis. A module displaying specific rule(s) given the diagnosis is also included along with modules for calculating prognostic scores for myelodysplastic syndromes by two different methods. Rapid look-up of leukocyte antigens (CD numbers) is also available. FABHELP has proven useful for teaching residents and as a quick, concise reference for FAB classification.