The number of existing cases of primary dystonia in the population is not precisely known, but the condition is probably much more frequent than reported. By minimum prevalence estimates, primary dystonia should be considered the third most frequent movement disorders after essential tremor and Parkinson's disease. The most likely etiologic scenario suggested by epidemiological data is that primary dystonias are products of a genetic background and an environmental insult. Current information on the causation of primary dystonia, late-onset dystonia in particular, is often unreliable because of methodological problems inherent to case-control investigation and to the heterogeneity of dystonia. To expand our knowledge on dystonia, we need to design population-based studies, to perform association studies taking into account the heterogeneity of dystonia, and to collect exhaustive clinical data in a standardized and reliable way.