Mood swings accompanying the motor fluctuations of patients with Parkinson's disease on chronic levodopa treatment frequently occur, but are poorly recognized. Occasionally, their functional impact may be greater than that caused by the motor disability itself. In this study we have assessed the nature of, and relationship between, mood and motor fluctuations in nine Parkinsonian patients with 'on-off' motor swings. The results of an additional questionnaire survey confirm that 'on-off' mood swings occur in approximately two thirds of patients with Parkinson's disease experiencing motor fluctuations on dopaminergic treatment. Aetiological and therapeutic implications are discussed.