Abstract In commercial growth of horticultural plants in greenhouses, high financial losses are being suffered due to the so-called thick root syndrome (TRS), a phenomenon characterized by severe deterioration of the root system. The early symptoms of TRS in cucumber ( Cucumis sativus L.) are strongly curving roots that are swollen and superficially damaged. Research focused on ethylene, as both the culture practice of cucumber and the known effects of ethylene on root growth point to a possible role for this phytohormone in TRS. Ethylene induced root curvature and swelling as well as damage of the epidermal layer and outer cortex of roots of cucumber plants similar to TRS symptoms. Formation of root hairs was stimulated and root elongation was also severely inhibited by exogenous ethylene. However, based on experiments with the ethylene inhibitor α-aminoisobutyric acid (AIB), a causal relation between ethylene and TRS in cucumber is doubted.