['Problems with language and symptoms of attention-deficit/hyperactivity disorder (ADHD) in childhood and adolescence are often strongly linked to low scholastic performance. Early recognition of children who are at increased risk is necessary. Our objective was to determine whether mixed-handedness, which is associated with atypical cerebral laterality, is associated with language, scholastic, and ADHD symptoms in childhood and adolescence.', 'Prospective data come from the Northern Finland Birth Cohort 1986, a longitudinal, population-based birth cohort with assessments when children were 7 to 8 and 16 years of age (N = 7871). Teacher, parent, and/or adolescent reports were used to assess language difficulties, scholastic performance, and mental health, including ADHD symptoms.', 'Mixed-handed children, relative to right-handed, had approximately a twofold increase in odds of having difficulties with language and scholastic performance at the age of 8 years. Eight years later, as 16-year-olds, adolescents had twofold increase in odds concerning difficulties in school with language and with ADHD symptoms. Mixed-handed children were more likely to have scores indicating probable psychiatric disturbance, including ADHD symptoms. As adolescents, mixed-handed children with previous behavioral problems were at considerably higher risk for scoring within the range of probable ADHD-inattention or ADHD-combined case. Mixed-handedness was associated with greater symptom severity in children and adolescents (P = .01) concerning psychiatric disturbance and ADHD inattention but not ADHD hyperactivity.'] Is mixed-handedness linked to mental health problems in children and adolescents?