Objective: Bipolar disorder (BD) is often a progressive mood disorder with a high prevalence of comorbid personality disorder (PD) ranging from 25 to 73 %. Previous studies have included patients with various illness duration of BD. Longer illness duration may be associated with increased prevalence of comorbid PD. This study investigated the prevalence of comorbid personality disorders in patients with newly diagnosed BD and their unaffected first-degree relatives (UR) compared with healthy control individuals (HC). Methods: We included 204 patients with newly diagnosed BD, 109 of their UR and 188 HC. To assess comorbid PD according to DSM-IV, the SCID-II-interview was performed in full or partial remission. Subthreshold PD was defined as scores above cut-off in the SCID-II self-report questionnaires. Functioning was assessed using the Functioning Assessment Short Test. Results: In total 52 (25.5 %) of the patients with newly diagnosed BD fulfilled criteria for a comorbid PD. Regarding UR, 7 (6.4 %) fulfilled the criteria for a PD. Subthreshold PD were more prevalent in BD (82.8 %) and UR (53.0 %) than in HC (35.1 %), p-values < 0.003). Patients with comorbid PD presented with impaired functioning compared with patients without PD. Limitations: Clinical diagnostic distinction between PD and BD is challenged by overlapping symptoms. Conclusion: A quarter of patients with newly diagnosed BD fulfill criteria for a comorbid PD, already at the time of the diagnosis with BD. A comorbid PD is associated with larger functional impairments. This emphasizes the need for early assessment of comorbid PD at time of BD diagnosis.