The absence of any menstrual flow by the age of 16. The median age for menarche is approximately 12.3 years of age. However, the range varies widely. Primary amenorrhea is not considered to have occurred until a girl is beyond age 16 years without the onset of menses. Primary amenorrhea may occur with or without other signs of puberty.

There are multiple causes for primary amenorrhea:

- normal delay of onset (up to age 14 or 15)
- drastic weight reduction (resulting from poverty, fad dieting, anorexia nervosa, bulimia, or other cause)
- congenital abnormalities of the genital system
- hypoglycemia
- extreme obesity
- gonadal dysgenesis
- Turner's syndrome (XO)
- hypogonadotropic hypogonadism
- testicular feminization syndrome
- true hermaphroditism
- chronic illness
- malnutrition
- Crohn's disease
- cystic fibrosis
- congenital heart disease (cyanotic)
- craniopharyngioma, ovarian tumors, adrenal tumors
- hyperthyroidism
- imperforate hymen, vaginal absence, cervical absence
- adrenogenital syndrome
- Prader-Willi syndrome
- polycystic ovarian disease
- congenital adrenal hypoplasia