- Macular degeneration—age-related
- Major histocompatibility complex
- Malignant hyperthermia
- Mannosidosis
- Marfan syndrome
- Marshall syndrome
- Marshall-Smith syndrome
- MCAD deficiency
- McCune-Albright syndrome
- McKusick-Kaufman syndrome
- Meckel-Gruber syndrome
- Meckel's diverticulum
- Menkes syndrome
- Metaphyseal dysplasia
- Methylmalonic acidemia
- Methylmalonicaciduria due to methylmalonic CoA mutase deficiency
- Microphthalmia with linear skin defects (MLS)
- Miller-Dieker syndrome
- Moebius syndrome
- Moyamoya
- Mucolipidosis
- Mucopolysaccharidoses
- Muir-Torre syndrome
- Multifactorial inheritance
- Multiple endocrine neoplasias
- Multiple lentigenes syndrome
- Muscular dystrophy
- Myasthenia gravis
- Myopia
- Myotonic dystrophy
