OBJECTIVES: Little is known about the aetiology of idiopathic adult onset dystonia. The Italian Movement Disorders Study Group promoted a case-control study on some hypothetical risk factors including past medical events, life events, life habits, occupational hazards, and family history of dystonia, parkinsonism, and tremor. METHODS: Cases affected by idiopathic adult onset dystonia (age at symptom onset >20 years, duration of disease >one year and <five years) were selected among consecutive outpatients attending 14 Italian centres. Control outpatients matched for age (+/-5 years), sex, and referral centre were identified among diagnostic categories thought to be unassociated with study exposures. Information was obtained by a standardised questionnaire administered by medical interviewers. Conditional logistic univariate and multivariate regression analyses were performed by a standard statistical package. RESULTS: Multivariate analysis on 202 cases and 202 age and sex matched control outpatients indicated that head or facial trauma with loss of consciousness, family history of dystonia, and family history of postural tremor independently increased the risk of developing adult onset dystonia, whereas hypertension and cigarette smoking exerted a protective effect. The findings also suggested a positive association between local body injury-for example, previous ocular diseases and neck or trunk trauma-and dystonia of the same body part. CONCLUSIONS: The results support the idea that environmental and genetic factors may both be important in the aetiology of adult onset dystonia, and suggest aetiological clues worthy of further analytical investigation.