The aim of this study was to show the diagnostic procedure used in the two cases with false-positive serum acetaminophen results in suspected acetaminophen poisoning. The determination of serum acetaminophen were carried out using a UV/VIS spectrophotometer (Specord 40 Analytik Jena), coupled with an analytic computer station WinASPECT. The employed method of determination was based on the acetaminophen reaction with sodium nitrite, which yields yellow colour of solution in the presence of sodium hydrate. The intensity of the yellow colour depends on the concentration of acetaminophen in serum. The relationship between absorbance and concentration was linear at concentrations in the range 50-600 microg/mL, with relative standard deviation of +/- 2.1% and detection limit of 30 microg/mL. To confirm or reject the doubtful results of colorimetric assays, the serums of patients were measured with high performance liquid chromatography with mass spectrometry detection and gas chromatography with mass spectrometry detection. The analysis of presented cases leads to a conclusion that acetaminophen results should be confirmed either by scanning urine for p-aminophenol presence (which is a routine procedure in our laboratory) or by using a different method of measuring acetaminophen serum levels.