We report the details of an accidental overdosage of haloperidol in 24 children in one hospital in Kyushu, Japan. Evidence of acute toxicity included disturbances in consciousness (24/24), tremors in the extremities (16/24), an oculogyric or similar crisis (14/24), dysarthria (9/17), drooling (8/24), akathisia (6/20), hyperreflexia (6/24) and opisthotonos (3/24). Laboratory examinations revealed late-onset transient thrombocytosis (5/24), elevated AST and GPT (1/24) and abnormal ECG with prolonged QT interval in 2 of 8 children. We detected haloperidol in 11 of 18 children whose blood was specifically examined within four days after the final haloperidol administration. The maximum serum haloperidol level was 28.9 ng/ml. The mean half-life of haloperidol in the serum of five children (age range 2-10 years) was 18.6 +/- 12.2 h (mean +/- SD) (range 9.1-39.4 h).