Tansy, Tanacetum macrophyllum (Waldst. & Kit.) Sch. Bip., is often misidentified by herb collectors as yarrow, Achillea grandifolia Friv. With the former, cases of poisoning induced by its ingestion are well documented, but the latter is widely used for ethnopharmacological purposes. The aim of this study was to estimate, based on the volatile metabolite profiles of the two species, the potential health risk connected with their misidentification. GC and GC-MS analysis of the essential oils hydrodistilled using a Clevenger-type apparatus from A. grandifolia, T. macrophyllum, and two plant samples (reputedly of A. grandifolia, but in fact mixtures of A. grandifolia and T. macrophyllum) obtained from a local market, resulted in the identification of 215 different compounds. The main constituents of A. grandifolia oil were ascaridole (15.5%), α-thujone (7.5%), camphor (15.6%), borneol (5.2%) and (Z)-jasmone (6.4%), and of T. macrophyllum oil, 1,8-cineole (8.6%), camphor (6.4%), borneol (9.1%), isobornyl acetate (9.5%), copaborneol (4.2%) and γ-eudesmol (6.2%). The compositions of the oils extracted from the samples obtained from the market were intermediate to those of A. grandifolia and T. macrophyllum. Significant differences in the corresponding volatile profiles and the literature data concerning the known activities of the pure constituents of the oils, suggested that the pharmacological action of the investigated species (or their unintentional mixtures) would be notably different. It seems, however, that misidentification of T. macrophyllum as A. grandifolia does not represent a health risk and that the absence of the toxic α-thujone from T. macrophyllum oil may in fact be regarded as a benefit.