In acute poisoning caused by acetaminophen (N-acetyl-p-aminophenol, APAP), it is critical to predict the onset of delayed liver injury based on the prompt measurement of serum APAP level and to administer the antidote N-acetylcysteine (NAC) without delay as needed. However, all emergency medical facilities are not necessarily equipped with an expensive analytical instrument that allows prompt determination of APAP. Here, we tested the clinical usefulness of the Acetaminophen Detection Kit^® (Kanto Chemical Co., Ltd.), which claims to rapidly detect APAP in serum using a simple procedure, by spectrophotometrically measuring the APAP concentration in 34 serum samples collected from 28 patients with acute APAP poisoning. The results showed that the correlation coefficient between the APAP value measured by the Acetaminophen Detection Kit^® and that determined by the HPLC method was, at 0.888, not very high, but that the decision on whether to administer NAC based on the measured APAP level was consistent between the two analytical methods in 23 out of 25 patients. Also, the value obtained by the Acetaminophen Detection Kit^® was equal to, or larger than, that obtained by the HPLC method, suggesting that it is unlikely that patients requiring NAC would be left untreated. These results indicate that the Acetaminophen Detection Kit^®, with its ease and simplicity of use, is clinically useful in emergency medical facilities for which an expensive analytical instrument is not affordable.