The chief basis for current diagnosis in toxicology is a history of adequate exposure to a chemical, associated with the presence of characteristic signs and symptoms. When both a clear history and clear signs are present, such a diagnosis is among the easiest in medicine. When either the history or signs are unclear or even lacking, such a diagnosis can be extremely difficult. Sometimes the patient, or those who accompany an unconscious patient, are simply unaware of an exposure which has occurred; or they may be aware of an exposure but completely uninformed about its significance. In such cases, a high index of suspicion on the part of the physician may lead to a correct diagnosis.