The essential oil composition of Acorns calamus (sweet flag) leaves collected in Lithuania at different growing phases was examined by GC and GC/MS. Rhizome oils of A. calamus, having been more thoroughly investigated, were used for comparison purposes. The content of the oil in dried sweet flag rhizomes was 1.20+/-0.12% and in the leaves, depending on the vegetation phase, was from 0.56-1.01%. Ninety-one constituents were positively or tentatively identified in the oils-66 in the leaves and 55 in the rhizomes. Possible formation of calacorene hydrates is suggested for the first time on the basis of mass spectral data. beta-Asarone [(Z)-asarone] was the major constituent in the leaves (27.4-45.5%), whereas acorenone was dominant in the rhizomes (20.86%) followed by isocalamendiol (12.75%). A higher content of some aliphatic and oxygenated monoterpenes was found in oils of the leaves at their earliest growth phase (May), while the beta-asarone content was at its lowest level.