Aromatase, a key enzyme in estrogen synthesis, is tissue-specifically regulated in various tissues and plays an important role through endocrine and intracrine estrogen production in various physiological functions. Therefore, aromatase deficiency caused crucial impairments of physiological functions in the gonadal tissues as well as extra-gonadal tissues. Because aromatase is protective for androgenic exposure by catabolizing, virilization of a pregnant mother and pseudohermaphroditism of a baby girl consequently result from the deficiency. Similarly, because aromatase is productive for a multifunctional physiological factor, estrogens, impaired metabolisms of bone, carbohydrate, and fat etc. result from the deficiency. We discuss the etiology, clinical symptoms, and therapeusis by classifying it into two types of complete and incomplete aromatase deficiencies.