The relationship between pneumatosis cystoides intestinalis (PCI) and occupation was studied in 66 patients reported in Japan during a 5-yr-period from 1979-1983. Information concerning their occupation was obtained from 37 (56.1%) patients; 16 patients had secondary PCI and 21 had primary PCI. No particular characteristics were noted in the occupations of secondary PCI patients. Primary PCI, occurring more frequently in females (15/21), affected predominantly the large intestine (20/21), locally involving the sigmoid colon in the majority (14) of the patients. Most patients with primary PCI (16/21) were factory workers, of whom 15 (71.4%) were engaged in degreasing of manufacturing products with trichloroethylene. The high percentage of trichloroethylene workers among the patients with primary PCI suggests that occupational exposure to this agent constitutes an etiological factor in the development of this disease.