A simple classification system of overdoses presenting to a hospital emergency department is described. Five major overdose categories were developed (accidental, suicidal behavior, recreational/experimentation, compulsive use, and an indeterminate category), which were divided into 10 subcategories. A prospective survey at an inner-city hospital in Sydney classified 199 consecutive self-poisoning presentations as being due to compulsive drug use in 38% of cases; suicidal gesture as a reaction to stress, 17%; adverse reactions to recreational or experimental use, 15%; suicidal gesture to influence others, 7%; genuine accident or noncompliance with medication, 6%; genuine suicide attempt, 3%; and pseudo-overdose, 3%. The reason for the overdose was uncertain in the remainder of the cases. Additional data are also presented, including the types of drugs involved in each overdose category. Categories of overdose may be useful in psychosocial assessment for follow-up interventions, assist in research, and help reduce negative attitudes toward overdoses.