OBJECTIVE: The aim of our study was to evaluate the incidence, duration and risk factors for benign neonatal sleep myoclonus (BNSM) in infants with neonatal abstinence syndrome (NAS) treated with opioids or sedatives, compared with control infants. METHODS: This is a single centre observational case control study. Seventy-eight near term and term infants with neonatal opiate abstinence syndrome confirmed by meconium analysis were included. Exclusion criteria were cerebral malformation, intracranial haemorrhage and perinatal asphyxia. The babies were assessed eight hourly with a modified Finnegan score that included sleep myoclonus. Seventy-eight infants not exposed to opiates during pregnancy, hospitalized for at least 14 days and matched for gestational age were used as controls. RESULTS: The median gestational age was 38 (1)/(7) (95% CI: 35 (3)/(7)-41 (2)/(7)) weeks, birth weight 2730 (95% CI: 1890-3600) g, umbilical artery pH 7.25 (CI 7.10-7.37) and Apgar score at 5 minutes 9 (95% CI: 7-10). The control infants did not differ in these characteristics. Sleep myoclonus was diagnosed in 52 (67%) of the infants with NAS and 2 (2.6%) of the controls (OR 26 [95% CI: 7-223], p < 0.001). Myoclonus appeared as early as day 2 and as late as day 56 of life (median day 6) and lasted for 1 to 93 days (median 13 days). All infants had serum glucose > 2.5 mmol/L at first occurrence. The neurological examinations as well as cerebral ultrasound scans were normal. An electroencephalogram (EEG) carried out in 18 infants showed no signs of epileptic activity. CONCLUSION: BNSM has a high incidence in infants with NAS. The diagnosis can be made clinically. In the absence of other neurological symptoms further investigations such as EEG are not necessary and anticonvulsive treatment is not indicated.