Summary: Purpose: Univerricht‐Lundborg disease (ULD), with its major symptom of action myoclonus, is supposed to be very rare in the Netherlands and western Europe. We hypothesized that the syndrome may be underdiagnosed in patients with myoclonus epilepsy. Methods: Mutation analysis of the cystatin B gene was performed in 21 cases with uncontrolled myoclonus. Results: Seven of the 21 evaluated cases carried mutations in the cystatin B gene. Diagnosis of ULD was made with a mean delay of 20 years from symptom onset. Conclusions: This study from a country without previous reports of ULD suggests that underdiagnosis of the syndrome is likely. These findings also indicate that persons with juvenile‐onset myoclonus epilepsy with action myoclonus should be analyzed for ULD.