The incidence of embolic complications among 426 patients with initial paroxysmal atrial fibrillation (PAF) was analysed. A distinct clustering of emboli was seen at the time of onset of PAF. After transition to chronic atrial fibrillation (CAF), which developed in 141 patients (33.1%), the incidence of emboli was seen to rise to a new level several times higher than the incidence level for patients with PAF. Also in this group a distinct clustering of emboli was seen during the first year after transition to CAF. On this background it is suggested that patients with PAF may benefit from treatment with anti-arrhythmic agents in order to prevent the development of CAF and that anticoagulants for stroke prevention seems especially desirable in atrial fibrillation (AF) of recent onset.