Background: While various guidelines on the treatment of bipolar disorder have been published over the last decades, adherence to guidelines has been reported to be low. In this article we describe the protocol of a nationwide, multicenter, longitudinal, non-intervention study on the treatment of bipolar disorder in the Netherlands. Study aims are to provide information on the nature and content of outpatient treatment of bipolar disorder, to determine to what extent treatment is in concordance with the Dutch guideline for the treatment of bipolar disorder (2008), and to investigate the relationship of guideline concordance with symptomatic and functional outcome. Methods/Design: Between December 2009 and February 2010, all psychiatrists registered as member of the Dutch Psychiatric Association received a questionnaire with questions about their treatment setting, and whether they would be willing to participate in further research. Psychiatrists treating adult outpatients with bipolar disorder were invited to participate. Consenting psychiatrist subsequently approached all their patients with bipolar disorder. The study is performed with written patient and caregiver surveys at baseline and after 12 months, including data on demographics, illness characteristics, organization of care, treatments received, symptomatic and functional outcome, quality of life, and burden of care for informal caregivers. Discussion: This study will provide information on the naturalistic treatment of bipolar disorder in the Netherlands, as well as degree of concordance of this treatment with the Dutch guideline, and its relationship with symptomatic and functional outcome. Limitations of a survey-based study are discussed.