The active (-) enantiomer of amlodipine was originally reported to have R configuration. This does not concur with other 1,4-dihydropyridines with known absolute configuration. This configuration has now been determined by X-ray structural analysis using (1S)-camphanic acid and (S)-2-methoxy-2-phenylethanol as chiral probes. Both determinations gave the S configuration for the amlodipine (-) enantiomer with the greater Ca-antagonistic activity.