Described is a hematology-diagnosis apparatus employing expert system technology. The apparatus comprises a computer/data processor which is provided with a memory facility, a central processor and a printer. Stored in the memory facility is a knowledge base applicable to hematologic diagnostics, and instructions by which the apparatus is to process input data, in the form of a plurality of--four in a specific exemplary embodiment--hematologic parametric numerics obtained from a patient's bood assay. The numerics are entered via entry means. The apparatus accordingly executes the instructions, and on the basis of the parametric numerics, prints out diagnostic and hematologic messages applicable to the patient.