The rhythm of vascular cambial activity was studied in four species of lianas growing in a lowland tropical rainforest of Mexico. Cambial activity was determined by counting the number of layers of cells in the cambial zone. A greater number of layers was assumed to indicate greater cambial activity. In all four species (Machaerium cobanense M. floribundum Gouania lupuloides and Trichostigma octandrum) the cambium is active throughout the year. In three of the species (all but T. octandrum) cambial activity was significantly higher in the April–October rainy period than in the November–March dry period. Cambial activity in Trichostigma octandrum was not significantly associated with the wet or dry season.