The occurrence of blossom‐end rot of tomatoes is correlated with a low Ca content. The disease incidence can be aggravated by water stress. An attempt was made to study the effects of water stress under conditions of equal concentration of ions by addition of polyethylene glycol in water culture. These treatments gave a higher incidence of blossom‐end rot and withering of the plant tops. Although direct effects of water stress on blossom‐end rot cannot be excluded a restricted uptake of most ions was demonstrated, Mg excepted. Calculation of the indexes (K+Mg)/Ca for uptake and for the leaf show that under influence of osmotic stress the relative Ca‐content is indeed lowered.