The results of an investigation of an apparent psychogenic illness outbreak among residents of the West Bank of Israel (the Arjenyattah epidemic) were summarized. An outbreak of symptoms including headache, dizziness, abdominal pain, blurred vision, and limb weakness occurred among West Bank residents in March/April 1983. In/home interviews and hospital physical and neurological evaluations were conducted with 114 school girls and ten adults with symptoms who lived at Arrabah and Yattah 2 to 12 days after onset of the symptoms. Fifty seven asymptomatic residents from the two villages served as controls. Blood and urine samples were analyzed for evidence of toxic exposures. Environmental air samples were collected at outbreak sites and analyzed. The physical and neurological examinations revealed tachycardia, mydriasis, and peripheral cyanosis in some affected subjects, but no other abnormalities. No antecedent exposures were identified. At Arrabah, eight subjects reported smelling an unusual odor before their symptoms began. At Yattah, half of the patients reported smelling an unusual odor. No significant differences in perceived antecedent health status or school performance were seen between the patients and controls. Analysis of the blood and urine samples revealed no detectable toxins. Hydrogen-sulfide (7783064) at concentrations up to 40 parts per billion (ppb) was detected inside the school at Arrabah. Sixteen to 50ppb hydrogen-sulfide was detected in a latrine located 10 meters from the main school building. Methane (74828) and total hydrocarbons at concentrations of 150 and 250 parts per million were also detected in the latrine. No other toxic environmental agents were detected. The authors conclude that the Arjenyattah epidemic was of psychological origin and probably triggered by the odor of low concentrations of hydrogen- sulfide escaping from a latrine at the school at Arrabah. Psychological stress and possibly news media publicity helped spread the epidemic.