The semiempirical method was applied to estimate the cytotoxic activity of 9 gallic acid derivatives against human promyelocytic leukemic HL-60 cells. Positive correlation was found between their 50% cytotoxic concentration (CC50) value and the heat of formation or total energy, but not between the CC50 value and size of the molecule (determined either by the length of the side chains, the largest distance of the molecule, the distance from R2 to carbonyl C or benzene ring C, or by Van der waals area). When CC50 value was plotted vs octanol/water partition coefficient (log P), parabolic curve, with a bottom at log P = 3-4, was produced, in a similar fashion to vitamin K, prenylflavanone and eugenol-related compounds. The present study suggests the usefulness of the semiempirical method to estimate the cytotoxicity of structurally related compounds.