A procedure for calculating a health index for oil-immersedtransformers using fuzzy set theory is introduced in this paper.Acidity, water content, the dissipation factor, and the BDV of thetransformer oil, in addition to furan analysis and DGA were usedas inputs for a fuzzy-logic model for calculating a transformerhealth index. Six-membership functions and 33 expert rules areused to map the six input values into output decisions for producing the transformer health index. The method presented wastested using field data for 90 working transformers. The resultswere then compared with the results obtained by a specializedAMHA. The comparison shows that 96.7% of the results aresimilar.The AMHA method was based on thresholds while themethod presented is based on memberships and is thereforemore general, and the rules used in the calculation are transparent to the user. Unlike the AMHA method, the presentedmethod can be used directly by utilities and industries in orderto determine the health indices of their fleets of transformers,which is a significant advantage.