Abstract Tramadol, racemic 1-(3-methoxyphenyl)-2-(dimethylaminomethyl)cyclohexane-1-ol, is an effective analgesic drug. Metabolites of tramadol described so far originate from O- and N-demethylation and are excreted in urine directly or after conjugation. A further metabolite was found in human liver microsome incubations and in the urine of volunteers after ingestion of tramadol. To elucidate the structure of the new metabolite, seven deuterated isotopomers of tramadol have been synthesized and ingested by volunteers. The mass spectra of the metabolites derived showed (i) that it was a hydroxy metabolite, (ii) that the hydroxy group was not located on the aromatic ring, the side chain, or the positions 2 and 6 of the cyclohexane ring, (iii) that the hydroxy-group was introduced to one of the the positions 3, 4 or 5 of the cyclohexane ring. The hydroxy metabolite was formed preferentially from the (-)-enantiomer, (1S,2S)-tramadol.