<s>[INST] <<SYS>> This is a medical question of type : frequency <</SYS>> How many people are affected by retroperitoneal fibrosis ? [/INST] Retroperitoneal fibrosis occurs in 1 in 200,000 to 500,000 people per year. The disorder occurs approximately twice as often in men as it does in women, but the reason for this difference is unclear. </s>