Emergency department data may underestimate deliberate self-harm if some records that are coded “undetermined” actually represent deliberate self-harm. Data from the National Ambulatory Care Reporting System were utilized. A total of 24,437 Ontario emergency department records for 2001/2002 coded deliberate self-harm or undetermined were analyzed. For every two emergency department presentations coded deliberate self-harm, another was coded undetermined. Cut/pierce injuries & poisonings coded undetermined appeared to represent deliberate self-harm more often than did undetermined presentations involving other injuries. (40 refs.)