Personality disorder not otherwise specified (PD NOS) is a frequently applied diagnosis, but we lack knowledge of the clinical appearance of patients receiving the diagnosis. This study applied a large clinical sample (N = 1516) to investigate (1) the prevalence and diagnostic and clinical characteristics of patients with PD NOS defined according to Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) and (2) the prevalence and psychosocial impairment associated with different definitions of PD NOS.Patients from the Norwegian Network of Psychotherapeutic Day Hospitals diagnosed in accordance with the LEAD standard (Longitudinal, Expert, All Data) were evaluated on several clinical measures at admission and discharge from short-term treatment.The DSM-IV PD NOS category was the third most frequent PD diagnosis. Seventeen percent of the total sample and 22% of those with PDs had a PD NOS diagnosis, with an average of 9 PD criteria. Patients with PD NOS were heterogeneous with respect to types of PD criteria, 41% were not subthreshold on any specific PD. The PD NOS patients were intermediate between patients with specific PDs and those with no PD regarding number of fulfilled PD criteria and several social and clinical variables. Alternative operationalizations of PD NOS, that is, subthreshold on at least 2 specific PDs or meeting a minimum of 10 PD criteria, resulted in lower prevalence rates and defined clinically more impaired patients.The PD NOS defined according to DSM-IV has a high prevalence and seems to capture a group of patients with fewer PD criteria and less severe psychosocial impairment compared to patients with specific PDs. The findings indicate that the operational definition of PD NOS may have a strong impact on the prevalence and clinical appearance of patient receiving this diagnosis.