The crystal structures of the following three stereoisomers have been determined: (1S)-2-[(1R,1'R)-, (1S)-2-[(1S,1'S)- and (1S)-2-[(1R,1'S)-bis(l-methylpropyl)amino]-1-[(5S)-1- [(2-chlorophenyl)methyl]-2-oxo-5-pyrrolidinyl]ethanol, C21H33ClN2O2 The configurations at the stereocentres strongly influence both the hydrogen-bonding behaviour of the OH group and the packing. The conformations of the central N-C-C(OH)-C-N chains are particularly relevant to this behaviour and to the pharmacological activity.