This study investigates the effects of aromatase deficiency in mice due to targeted disruption of the *cyp19* gene, which encodes aromatase cytochrome P450 (P450arom). Mice lacking functional aromatase (ArKO) were generated and their phenotypes were examined. Female ArKO mice at 9 weeks of age showed underdeveloped external genitalia and uteri, with numerous follicles containing abundant granulosa cells but arrested antrum formation and no corpora lutea. The stroma was hyperplastic with structures resembling atretic follicles. Mammary gland development was similar to that of prepubertal females. Male ArKO mice had enlarged accessory sex glands due to increased secreted material content but normal internal anatomy and fertility. Serum estradiol levels were undetectable, while testosterone levels were elevated, along with follicle-stimulating hormone (FSH) and luteinizing hormone (LH) levels. The phenotype of ArKO mice differs from that of estrogen receptor α (ERKO) mice, where the estrogen receptor is deleted. The findings highlight the importance of estrogens in the development and function of reproductive organs and other tissues, and suggest potential roles for estrogen in male and female physiology and pathophysiology.This study investigates the effects of aromatase deficiency in mice due to targeted disruption of the *cyp19* gene, which encodes aromatase cytochrome P450 (P450arom). Mice lacking functional aromatase (ArKO) were generated and their phenotypes were examined. Female ArKO mice at 9 weeks of age showed underdeveloped external genitalia and uteri, with numerous follicles containing abundant granulosa cells but arrested antrum formation and no corpora lutea. The stroma was hyperplastic with structures resembling atretic follicles. Mammary gland development was similar to that of prepubertal females. Male ArKO mice had enlarged accessory sex glands due to increased secreted material content but normal internal anatomy and fertility. Serum estradiol levels were undetectable, while testosterone levels were elevated, along with follicle-stimulating hormone (FSH) and luteinizing hormone (LH) levels. The phenotype of ArKO mice differs from that of estrogen receptor α (ERKO) mice, where the estrogen receptor is deleted. The findings highlight the importance of estrogens in the development and function of reproductive organs and other tissues, and suggest potential roles for estrogen in male and female physiology and pathophysiology.