Psidium guajava L. (Myrtaceae) has been used traditionally against gastrointestinal disturbances and respiratory ailments. The chemical composition of the essential oil of both leaves and fruits were elucidated by gasâliquid chromatography/mass spectrometry (GLC/MS). Forty-five and forty-two compounds, accounting for 93.7% and 89.7% of the fruit and leaf oil, were identified, respectively. The dominant compounds were β-caryophyllene (17.6%) and limonene (11.0%) for the fruit oil and β-caryophyllene (16.9%) and selin-7(11)-en-4α-ol (8.3%) for the leaf oil. The radical scavenging activities of both essential oils were assessed by the diphenyl picrylhydrazyl (DPPHâ¢) and deoxyribose degradation assays. Guava leaf oil reduced DPPHâ¢ radicals and prevented the degradation of the deoxyribose with ic50 values of 3.59 and 12.64 μg/mL. The in vitro cytotoxicity of the oils in HepG2 and MCF-7 carcinoma cells was examined using the SRB assay (ic50 32.53 and 49.76 μg/mL for the leaves and fruit oils against HepG2 cells). Inhibition of 5-lipoxygenase (5-LOX) was used to evaluate the anti-inflammatory activity of both oils (ic50 130.69 and 196.45 μg/mL for the leaves and fruit oils). The anti-inflammatory activity was explained via virtual docking of the major identified compounds to the main sites in the 5-LOX crystal structure.