The 21-hydroxyl group of cortisol was selectively acetylated under mild conditions without affecting the more sterically hindered 11 beta- and 17 alpha-hydroxyl groups. The reaction was performed with a mixture of acetic anhydride and triethylamine in acetonitrile and was complete in less than 15 min at room temperature. The thermospray mass spectrum of cortisol 21-acetate showed minimal fragmentation with the [M + H]+ ion as the base peak. High sensitivity was achieved for acetylated cortisol during selected ion monitoring, the signal-to-noise ratio being increased by a factor of about 4, compared to underivatized cortisol. The limit of detection of cortisol 21-acetate was estimated at 0.24 pmol injected, making thermospray liquid chromatography-mass spectrometry competitive with gas chromatography-mass spectrometry for the determination of cortisol in biological fluids.