| X-Linked Agammaglobulinemia | Gale Encyclopedia of Medicine |
| X-linked hydrocephaly | Gale Encyclopedia of Genetic Disorders Part I |
| X-Linked Hydrocephaly | Gale Encyclopedia of Genetic Disorders Part II |
| X-Linked Mental Retardation | Gale Encyclopedia of Genetic Disorders Part II |
| Xanthoma | ADAM Health Illustrated Encyclopedia |
| Xeroderma pigmentosa | ADAM Health Illustrated Encyclopedia |
| Xeroderma pigmentosum | Gale Encyclopedia of Genetic Disorders Part I |
| Xeroderma Pigmentosum | Gale Encyclopedia of Genetic Disorders Part II |
| Xerophthalmia | Gale Encyclopedia of Nutrition and Well Being |
| Xerosis | ADAM Health Illustrated Encyclopedia |
| Xerostomia | Gale Encyclopedia of Cancer |
| XX male syndrome | Gale Encyclopedia of Genetic Disorders Part I |
| XX Male Syndrome | Gale Encyclopedia of Genetic Disorders Part II |
| XYY syndrome | Gale Encyclopedia of Genetic Disorders Part I |
| XYY Syndrome | Gale Encyclopedia of Genetic Disorders Part II |