Psychiatric hospital treatment (PHT) is expensive and indicates a severe disorder. Investigation of the early identification of this small patient group has though been hindered by small samples or unsatisfactory assessment in childhood. The present study aims to study the predictive association between psychopathology at age 8 using multi-informant assessment and later PHT. A nationwide birth cohort of Finnish children (n = 5,346) was assessed at age 8 to obtain information about psychopathology using the Rutter parent and teacher reports and self-reports of depressive symptoms. The main outcome was admission to any hospital with a primary diagnosis of any psychiatric disorder according to the Finnish National Hospital Discharge Register between age 13 and 24. Between age 13 and 24, 6.2% of the males and 4.1% of the females had been admitted for PHT. Among males, PHT was independently predicted by non-intact family and adult reports of conduct and of emotional symptoms, while among females by self-reported depressive symptoms. However, the combination of conduct and emotional problems was the strongest predictor for PHT in both sexes. Admission due to psychosis among males was associated with childhood conduct, attention, and emotional problems, but with emotional problems among females. Psychopathology at age 8 can be seen as a long-lasting increased risk of severe psychiatric disorders requiring hospital treatment in adolescence or early adulthood. Attention should be paid to self-reports among females and of comorbid conduct and emotional problems in both sexes in the early identification of this patient group.