Introduction Bipolar I disorder is a mental disorder with the risk of severe clinical outcomes. Bipolar disorder was initially defined based on having a better outcome than schizophrenia. However, while recent longer-term findings in schizophrenia do not support neuroprogression, bipolar disorder is increasingly depicted as having neuroprogressive elements. There are, however, remarkably few prospective longitudinal studies of representative bipolar I cohorts followed from the first treatment. Objectives To study the clinical development of a representative cohort of bipolar disorder patients recruited at their first treatment. Methods Patients with DSM-IV Bipolar I or Bipolar NOS were consecutively recruited from in-and outpatient units in the larger Oslo area during their first treatment year and extensively clinically characterized at baseline. They then participated in personal one- and ten-year follow-ups. Results Sixty-nine patients participated in the 10-year follow-up. Age at follow-up was 39.0 ( + 9.6) years, 59% were females. A total of 12% had unipolar mania, 58% had psychotic bipolar disorder, and 20% had experienced rapid cycling. At follow-up, 75% were in full affective remission, 60% had regained full functioning, and 54% were in stable full recovery. Mood episode relapses clustered around the first episode. Despite occasional relapses, 2/3 were mainly euthymic during the follow-up period. A small sub-group was highly affected from the first 2-3 years of treatment, but there were no apparent signs of kindling effects or indications of neuroprogression Conclusions The follow-up of this cohort of first-treatment Bipolar I patients does not support the hypothesis of neuroprogression. Disclosure of Interest None Declared