The essential oils (EOs) obtained from the leaves of Iryanthera polyneura Ducke trees was chemically Assessed and tested for the ability of inhibiting the growth of Candida albicans, Enterococcus faecalis, Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus, Streptococcus mutans and S. sanguinis. The oil was also tested against breast (MCF-7) and prostate (PC-3) cancer cell lines. Minimum bactericidal concentrations (MBCs) and 50 % inhibition concentrations (IC50) values were obtained. EOs were active against Gram-positive bacteria. Spathulenol, α-cadinol and τ-muurolol were major components of EOs. The oils showed a higher cytotoxicity against PC-3 than MCF-7 cells, although the oils were active against both cell types. Oils obtained from leaves collected in the dry season were more active against E. faecalis, S. aureus and PC-3, while the oils obtained from leaves collected in the rainy season were more active against S. mutans, S. sanguinis and MCF-7. The antibacterial and cytotoxic activities of the essential oils from the leaves of I. polyneura are related to the seasonal climate variation and are influenced by compounds that are minor components of the oils.