The issue of mental health in children with mental retardation began to be addressed more closely after the recent introduction of the term »dual diagnosis« denoting the coincidence of mental illness and mental retardation. At present, there is still much vagueness regarding the recognition of mental illness, possible causes and factors influencing the development of mental illness and the medical treatment used. The aim of the study was a) to determine possible differences in physical and social characteristics between children with mental retardation and those without intellectual disabilities, b) to investigate the impact of various demographic, physical and social factors on the level of mental retardation, c) to determine the incidence of mental illnesses among children with mental retardation and the affects of demographic, physical and social factors on these illnesses. Our study involved 46 children with mental retardation and 50 children without intellectual disabilities. Signs of emotional and behavioural disturbances were identified using the Developmental Behaviour Checklists completed by class teachers. The results of the study have confirmed the value of Developmental Behaviour Checklist for clinical work and research. The correlations observed speak for the coincidence and interdependence of physical occurrences, social influences, mental retardation and mental illness in the child’s development.