Four cases of primary pneumatosis cystoides intestinalis (PCI) were studied in relation to occupational trichloroethylene (TRI) exposure. Three of the patients were exposed to TRI in the process of washing or polishing of camera lenses. The maximum concentration of TRI in the work environment of two PCI patients often exceeded 50 ppm, though the geometric mean concentrations were below this value. Monitoring of TRI exposure by measuring urinary TRI metabolites suggested that the third patient had been working in the environment with an 8-h time weighted average TRI concentration exceeding 50 ppm. Scrutiny of 19 PCI cases thus far collected by us revealed that i) three groups of patients (7 in total) were working in the same factories, ii) most of the PCI patients were workers in small-scale factories having poor working conditions, and iii) occupational TRI exposure is probably responsible for the development of primary PCI.