Noon stomatal conductance of lethal-yellowing (LY) affected coconut palms (Cocos nucifera L. Atlantic Tall ecotype) decreased progressively as the disease developed. In the early stages of the disease and before leaf yellowing started, stomatal conductance decreased to a minimum in leaves at both the top, the middle and the base of the crown. Since altered stomatal behaviour might affect gas exchange and related processes in the plant, such as water movement and photosynthesis, these results support the hypothesis that LY-induced stomatal closure is central to the development of LY symptoms in coconut palms, and therefore to the mode of pathogenicity of the LY mycoplasma-like organism. Leaf yellowing occurred simultaneously with a decrease in photosynthetic rates and a decrease in protein, chlorophyll and carotenoid contents. Based on these biochemical changes a second hypothesis is proposed, that LY-associated leaf yellowing is part of a leaf senescence process. The concentration of abscisic acid in the leaves and the capacity of leaf tissue to form ethylene increased in diseased palms, suggesting that a hormonal imbalance occurs in LY-affected palms. Furthermore, treatment of symptomless palms with an ethylene-releasing agent (ethephon) resulted in symptoms that mimicked some of the LY symptoms. A third hypothesis is therefore proposed, that a hormonal imbalance might be related to LY symptom development, at least with respect to nutfall and leaf senescence.