The amount and composition of the essential oil from leaves of Hypericum androsaemum L. cultivated in Arouca (Portugal) were determined in six samples harvested during 1 year at intervals of 2 months. The seasonally dependent essential oil content ranged from 0.7 mg/g biomass dry weight in September to 3.4 mg/g in February. The oil contained more than 80 compounds, 70 of which (constituting 88-93% of the total oil) were identified by GC and GC-MS. An approximation of the absolute quantification of each compound and compound class was performed using a GC method with an internal standard. The relative and the absolute content of each compound and compound class changed during the year. At the end of the winter and in the spring, the essential oil was dominated by sesquiterpene hydrocarbons and accumulated a high number of intermediate to long chain n-alkanes and 1-alkenes. In September, the essential oil contained the lowest levels of sesquiterpene hydrocarbons (43%) and the highest levels of 1-octene and 2-hexenal (38%). In February, the essential oil had the highest level of sesquiterpene hydrocarbons (73%) and the highest diversity of intermediate to long chain n-alkanes and 1-alkenes.