MORE than 30 descriptions have been published of tissues excised at laparotomy from victims of periodic peritonitis. Except for a few details, the lesions were similar and unspecific. At laparotomy between episodes, the peritoneum was normal or showed evidence of subsiding inflammation and scars or adhesions as results of many previous bouts. During an episode, sterile fluid and flecks of exudate often were present. The serosa of the mesentery, diaphragm, appendix, and visceral organs was edematous, congested, or inflamed, with occasional petechiae. Fresh and old, dense or thin adhesions and scars usually existed. Microscopically, the vessels were dilated and engorged. Varying proportions of lymphocytes, monocytes, plasmacytes, neutrophils, and eosinophils invaded the serosa, subserosa, and occasionally the muscularis. Sections or blocks of tissue from 8 patients were obtained from pathologists for restudy, but no further information accrued concerning serosal lesions. As a new observation, hepatitis was present in 2 patients.