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In this article, the authors discuss 9-hydroxymethyl-10-carbamoyl acridan (9-OH-CBZ), another metabolite of carbamazepine (CBZ) found in serum. The retention time of unconjugated 9-OH-CBZ should be known when using a chromatographic method, because it appears in concentrations varying from one eighth to one third of the CBZ-10,11-epoxide (CBZ-E) concentration and may therefore cause analytical interactions. Liquid chromatography/electrospray mass spectrometry ((LC/ES-MS) in a serum extract identified and confirmed 9-OH-CBZ. The amount of 9-OH-CBZ present as conjugate in serum was between 42% and 65%. The correlation factor values (r) between serum concentrations of 9-OH-CBZ and 10,11-dihydro-10,11-trans-dihydroxy-CBZ (trans-CBZ-diol), CBZ-E, and CBZ in 100 serum samples were 0.77, 0.80, and 0.53, respectively. The origin of 9-OH-CBZ is discussed.