In most societies, mothers are the primary providers of nutrition and care to young children. This is a demanding task, and poor physical or mental health in mothers might be expected to have adverse consequences on their children's health, nutrition and psychological well-being. Child nutrition programmes do not adequately address maternal mental health. In this article, we consider the evidence from less developed countries on whether maternal mental health influences child growth, with respect to evidence from both observational studies and from clinical trials. We estimate how much of the burden of undernutrition might be averted in one setting, and propose that promoting maternal mental health and treating maternal mental illness offer important new opportunities to tackle the twin scourges of maternal ill-health and child undernutrition.