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An orchidometer is a medical instrument used to measure the volume of the testicles.

The orchidometer was introduced in 1966 by pediatric endocrinologist Andreas Prader of the University of Zurich. It consists of a string of twelve numbered wooden or plastic beads of increasing size from about 1 to 25 millilitres. Doctors sometimes informally refer to them as "Prader's balls", "the medical worry beads", or the "endocrine rosary."

The beads are compared with the testicles of the patient, and the volume is read off the bead which matches most closely in size. Prepubertal sizes are 1–3 ml, pubertal sizes are considered 4 ml and up and adult sizes are 12–25 ml.

The orchidometer can be used to accurately determine size of testes. Discrepancy of testicular size with other parameters of maturation can be an important clue to various diseases. Small testes can indicate either primary or secondary hypogonadism. Testicular size can help distinguish between different types of precocious puberty. Since testicular growth is typically the first physical sign of true puberty, one of the most common uses is as confirmation that puberty is beginning in a boy with delay. Large testes (macroorchidism) can be a clue to one of the most common causes of mental retardation, fragile X syndrome.

Professor Stephen Shalet, a leading endocrinologist who works for the Christie Hospital in Manchester, is reported to have told The Observer, "Every endocrinologist should have an orchidometer. It's his stethoscope."

• Prader, A., "Testicular size: Assessment and clinical importance", Triangle, 1966, vol. 7, pp. 240 - 243
• Taranger, J., Engström, I., Lichtensten, H., Svenberg-Redegren, I., "Somatic Pubertal Development", Acta Pediatr. Scand. Suppl. 1976, vol. 258, pp. 121 - 135

• "Orchidometer" - A printable paper orchidometer in PDF format.