BACKGROUND: Mild traumatic brain injuries (mTBIs) are common in childhood and adolescence, but their long-term effects remain poorly understood. OBJECTIVE: Examine cognitive performances and perceived injury symptoms, on average, six years after an mTBI in school-aged children and adolescents, and to evaluate longitudinal changes in performance and symptoms during the follow-up period. MATERIALS AND METHOD: Finnish children aged 7-15 years who were cared for mTBI at Turku University Hospital during 2010-2016, with brain imaging and neuropsychological assessment linked to the event available, were identified and targeted for follow-up assessment. We gathered cognitive performance and injury symptom data, as recorded at 1-3 months post-injury, retrospectively from the hospital patient records. Age-appropriate versions of Wechsler Intelligence Scale, Conners Continuous Performance Test II, PedsQL™ Multidimensional Fatigue Scale and semi-structured interview of symptoms were used as outcome measures at follow-up. RESULTS: Age-adjusted verbal performance scores of the participants deteriorated during follow-up, and were predicted by younger age at injury, male sex, and lower verbal performance scores at the original assessment. At follow-up 64.9 % reported one or more injury symptoms, with 48.6 % of those displaying symptoms at the original assessment continuing to report symptoms at follow-up. The most persistent injury symptoms were verbal difficulties, headache and fatigue. CONCLUSIONS: The results stress the importance of identifying and monitoring children recovering slowly after a hospital-treated mTBI, as they might be at increased risk for long-lasting problems.