Objective: At least 50% of bipolar disorder (BD) patients have an additional diagnosis, one of the most difficult to manage being obsessive-compulsive disorder (OCD). Defining the nosology of BD-OCD comorbidity has important clinical implications, given that treatments for OCD can worsen BD outcomes. Method: A systematic review was conducted on: i) BD-OCD comorbidity lifetime prevalence and ii) on standard diagnostic validators: phenomenology, course of illness, heredity, biological markers, and treatment response. Relevant papers published through March 30th 2013 were identified searching the electronic databases MEDLINE, Embase, PsycINFO, and the Cochrane Library. Results: Sixty-four articles met inclusion criteria. Lifetime comorbidity prevalence was 11-21% in BD patients and 6-10% in OCD patients. Compared to non-comorbid subjects, BD-OCD has a more episodic course of OC symptoms (up to 75% vs. 3%), typically with worsening during depression (78%) and improvement during mania/hypomania (64%), as well as a higher total mean number of depressive episodes (8.9 ± 4.2 vs. 4.1 ± 2.7) and perhaps more antidepressant-induced mania/hypomania (39% vs. 9%). Conclusion: In this first systematic review of BD-OCD comorbidity, it appears that OC symptoms are usually secondary to BD, rather than representing a separate disease. © 2014 John Wiley & Sons A/S.