Many colleagues who have worked in casualty departments or emergency rooms will have encountered anxious parents bearing fragments of glass thermometers that do not include the mercury-containing bulb, and whose children are presumably suffering from indigestion or worse. Many of those colleagues will have found the parental anxiety infectious. Mercury is after all known to be a very toxic substance. An X-ray of the abdomen reveals the offending object. What is to be done? Purge? Operate? Chelate? Or is masterly inactivity indicated? The condition associated with acute and chronic mercury poisoning is named mercurialism. It is a combination of neurological symptoms and signs that include an erratic paranoid behavioural disturbance named erethism. Perhaps the proverbial mercurial personality is not a metaphor for mercury rising and falling in a thermometer tube, but rather refers to erethism. Renal and haematological effects are also described.