There is a genetic contribution to the idiopathic restless legs syndrome (iRLS). An autosomal dominant mode of inheritance is suspected, but as yet no gene has been identified. To assess the frequency and characteristics of the hereditary restless legs syndrome (RLS) in comparison to those of non-hereditary RLS, we analysed the clinical data of 300 RLS patients. All 300 patients diagnosed as RLS according to the criteria of the International RLS Study Group were examined using a standard questionnaire covering demographic data, family history, clinical symptoms, subjective sleep disturbances and course of the disease. In all patients a complete neurological examination was performed, and in selected cases electrophysiological examinations and polysomnographic studies. Family history was rated as definitely positive when at least one first-degree relative was examined and classified as RLS according to the criteria by one of the authors. If it proved impossible to contact family members to verify reports of a family history, the patients were classified as only having a "possible positive family history." 232 of the 300 patients had iRLS and 68 secondary RLS due to uremia (uRLS). 42.3% of the patients with iRLS and 11.7% of those with uRLS were classified as having "definite positive" hereditary RLS, with a further 12.6% of iRLS patients and 5.8% of uRLS patients as having "possible positive" hereditary RLS. Patients with definite hereditary RLS were significantly younger at the age of onset than those with a negative family history (35.45 vs. 47.17 years, p < 0.05). The clinical characteristics of the disease were similar in both groups, except that women with hereditary RLS experienced a worsening of symptoms during pregnancy (19.1% vs. 2.6%, p < 0.05). Our study shows that patients with hereditary RLS may experience an earlier onset of the disease. Hereditary and non-hereditary RLS present with similiar clinical signs and symptoms.