We recently showed a beneficial effect of selective dorsal rhizotomy (SDR) on daily care and comfort in nonwalking children with severe bilateral spasticity. However, despite careful selection, some patients showed dystonia after the intervention, in which cases caregivers tended to be less satisfied with the result. The aim of this study is to identify risk factors for dystonia after SDR in children and adolescents with severe bilateral spasticity (GMFCS levels IV/V). Clinical and MRI risk factors for dystonia after SDR were studied in our cohort of 24 patients. Patients with clinical evidence of dystonia and brain MRI showing basal ganglia abnormalities were excluded for SDR. Nine of 24 patients (38%) showed some degree of dystonia after SDR. There was a significant association between the cause of spasticity and dystonia after SDR; in six (67%) patients with a congenital disorder, dystonia was present versus three (20%) with an acquired disorder (Chi-squared test: C(1) = 5.23, p = 0.02). This study allows more optimal selection of patients that may benefit from SDR. Patients with an acquired cause of spasticity, when selected carefully on clinical examination and MRI, rarely show dystonia after SDR. However, patients with an underlying congenital disorder have a considerable risk of dystonia after SDR.