- X-ray (of)
- Xanthelasma (eyelid) (palpebrarum) H02.60
- Xanthelasmatosis E78.2 (essential)
- Xanthinuria, hereditary E79.82
- Xanthoastrocytoma
- Xanthofibroma – see Neoplasm, connective tissue, benign
- Xanthogranuloma D76.3
- Xanthoma(s), xanthomatosis (primary) (familial) (hereditary) E75.5
- Xanthosis R23.8
- Xenophobia F40.10
- Xeroderma – see also Ichthyosis
- Xerophthalmia (vitamin A deficiency) E50.7
- Xerosis
- Xerostomia K11.7
- Xiphopagus Q89.4
- XO syndrome Q96.9
- XXXXY syndrome Q98.1
- XXY syndrome Q98.0