The BIO study aims to identify (1) a composite blood-based biomarker, (2) a composite electronic smartphone-based biomarker and (3) a neurocognitive signature for bipolar disorder. There are three cohorts of participants included in this study: almost 390 patients diagnosed with bipolar disorder, 150 unaffected first-degree relatives of those with bipolar disorder in the study, and 200 healthy control participants. Participants diagnosed with bipolar disorder and their relatives were recruited from the Copenhagen Affective Disorder Clinic in Copenhagen, Denmark. Healthy controls were recruited from the blood bank at Rigshospitalet in Copenhagen. Participants completed baseline assessment from 2017 to 2021. Patients and healthy control participants will be assessed at least every year during the first four years and then every second year for five years. There are four sub-studies included in BIO investigating blood-based biomarkers, smartphone-based biomarkers, neurocognitive and brain imaging signatures, and prodromal phase of bipolar disorder.