The invention relates to the pharmaceutical use of specific substance P antagonists, in particular 1-acylpiperidine substance P antagonists, especially N-benzoyl-2-benzyl-4-(azanaphthoyl-amino)-piperidines, e.g. of formula wherein X and Y are each independently of the other N and/or CH and the ring A is unsubstituted or mono- or poly-substituted by substituents selected from the group consisting of lower alkyl, lower alkoxy, halogen, nitro and trifluoromethyl; and pharmaceutically acceptable salts thereof for treatment of chronic fatigue syndrome (CFS) in the absence of serotonin agonist/selective serotonin reuptake inhibitory therapy, or for the treatment of fibromyalgia or associated functional symptoms.