In December, 1984, in Bhopal, India, a massive leak of methyl isocyanate (MIC) resulted from operational and equipment malfunctions in a pesticide plant. Many thousands of residents of the city, most in proximity to the plant, suffered sublethal and lethal respiratory injuries, the expected consequences of high-level exposure to this type of potent irritant chemical vapour. Animal toxicologic information was limited prior to the accident, but has since confirmed that the lung is the major target of these lethal injuries, invariably with pulmonary oedema. Early concerns regarding acute cyanide intoxication were not supported by subsequent scientific inquiry. Superficial corneal erosions did not result in permanent eye injury. The primary medical (and, presumably, legal) issue which is unresolved, and perhaps unresolvable, is the incidence and determinants of long-term respiratory injury in the survivors. Available evidence, which is limited, suggests that chronic damage, when present, is, or resembles, fibrosing bronchiolitis obliterans, the expected consequence when permanent injury results from acute, high-level irritant gas exposure. Definition of the follow-up population is uncertain, and exposure information is lacking. Dose-response relationships are not likely to emerge from follow-up studies.