PURPOSE: Developmental stuttering may be associated with diminished psychological well-being which has been documented from late childhood onwards. It is important to establish the point at which behavioural, emotional and social problems emerge in children who stutter. METHODS: The study used data from the Millennium Cohort Study, whose initial cohort comprised 18,818 children. Analysis involved data collected when the cohort members were 3, 5 and 11 years old. The association between parent-reported stuttering and performance on the Strengths and Difficulties Questionnaire was determined in regression analyses which controlled for cohort members’ sex, verbal and non-verbal abilities, maternal education, and family economic status. RESULTS: Compared with typically-developing children, those who stuttered had significantly higher Total Difficulties scores at all three ages; in addition, scores on all of the sub-scales for 5-year-olds who stuttered indicated poorer development than their peers, and 11-year-olds who stuttered had poorer development than peers in all areas except prosocial skills. At ages 5 and 11, those who stuttered were more likely than peers to have scores indicating cause for clinical concern in almost all areas. CONCLUSION: Children who stutter may begin to show impaired behavioural, emotional and social development as early as age 3, and these difficulties are well established in older children who stutter. Parents and practitioners need to be aware of the possibility of these difficulties and intervention needs to be provided in a timely fashion to address such difficulties in childhood and to prevent the potential development of serious mental health difficulties later in life.