In this paper, the theory of fuzzy sets and fuzzy logic is approached as a diagnostic aid for practitioners and biologists. A few theoretical considerations are followed by two practical applications. The first is a modification of the WHO-Fredrickson classification of hyperlipoproteinemias, with the introduction of secondary or minor types corresponding to everyday medical practice. The other is an attempt at classifying the four most prevalent anemias encountered in internal medicine. Recent studies have shown that the various parameters should be grouped on a hierarchical basis.