Medullary cystic disease is a disorder involving the formation of cysts in the kidney medulla. The condition is inherited in most cases. The disorder causes an inability to concentrate the urine.

This results in excessive fluid loss from the body, which is accompanied by loss of sodium. This is a "high output" type of chronic renal failure (renal failure with increased urine volume, but with all other aspects of decreased kidney function).

The disorder is rare. This disorder causes end-stage renal disease in less than 5% of the pediatric end-stage renal disease population. The disorder runs in families, and the inheritance patterns vary.

There is X-linked and autosomal dominant inheritance in different families. X-linked inheritance is carried on the X chromosome, and is expressed in males that inherit the faulty gene from their mother. It is expressed in females that inherit the faulty gene from both parents. Females that only inherit the gene from one parent will not develop the disorder, but can pass it on to their children. With autosomal dominance, if one parent has the gene for the disorder, 50% of the children will display symptoms.

Risk factors include an individual or family history of medullary cystic disease, chronic renal failure, or increased urinary volume with excessive thirst.