OBJECTIVE: To report the first 2 European cases of biotin-responsive basal ganglia disease and novel SLC19A3 mutations. DESIGN: Case reports. SETTING: University hospital. Patients A 33-year-old man and his 29-year-old sister, both of Portuguese ancestry, presented with recurrent episodes of encephalopathy. Between episodes patients exhibited generalized dystonia, epilepsy, and bilateral hyperintensities of the caudate and putamen. MAIN OUTCOME MEASURES: Clinical and radiologic findings. RESULTS: Administration of high doses of biotin or of a combination of biotin and thiamine during encephalopathies resulted in spectacular clinical and radiologic improvement in both patients. Sequencing of the SLC19A3 disclosed 2 novel mutations, both of which created premature stop codons in the protein sequence of hTHTR2. CONCLUSION: This study demonstrates that biotin-responsive basal ganglia disease is a panethnic condition. A therapeutic trial with high doses of biotin and thiamine seems mandatory in every unexplained encephalopathy with bilateral lesions of putamen and caudate nuclei.