Children with mild developmental delay without dysmorphic features do not often have identifiable underlying aetiological factors. We report on a 5-year-old girl with mild developmental delay and dysmorphic features which were previously unrecognized. She was found to have supernumerary ring chromosome 19 mosaicism which was the likely cause of her clinical problems. Her parents' chromosomes were normal. A careful examination for dysmorphic features should be done in all children with developmental delay. However, these may not be readily apparent in babies and very young children. Chromosomal analysis to identify a genetic cause and to offer genetic counselling should be considered in all such children unless the clinician is absolutely certain that there are no dysmorphic features.