There are various conditions in JET under which the energy confinement of aplasma is reduced from H mode towards L mode levels. Examples are reported here forwhich the loss of confinement is associated with a reduction of the electron pressurepedestal. This loss is brought about by strong gas puffing that increases the frequencyof edge localized modes (ELMs). The effect of ELMs on confinement is modelled, and usedto explain the observed degradation of confinement. The model is generalized forapplication to other machines.