Background: Mass psychogenic illness refers to outbreaks of illness attributed to a toxic agent but for which no plausible organic cause is found. We determined the frequency and predictors of mass psychogenic illness within a sample of chemical incidents. Methods: Information was collected on a random sample of 280 chemical incidents. We developed consensus operational criteria for mass psychogenic illness and estimated its frequency. We then assessed environmental, emergency, and health service indicators for their association with mass psychogenic illness. Results: Nineteen "chemical incidents" were probable episodes of mass psychogenic illness. This represented 16% of incidents for which people reported symptoms and 7% of all incidents. Odor was a robust predictor of mass psychogenic illness. These illnesses were especially likely to occur in schools or healthcare facilities. Conclusions: A substantial minority of chemical incidents may be mass psychogenic illness.