The Beck Cognitive Insight Scale (BCIS) consists of two subscales measuring Self-Reflectiveness and Self-Certainty. The present study investigated the relationship of the BCIS and poor clinical insight. Moreover, the associations of cognitive insight with subjective psychotic experiences and depression/suicidality were investigated. A schizophrenia inpatient sample (SCH, N = 88) was compared to a mixed sample of non-psychotic inpatients, most of them with unipolar depression (MIX, N = 85), and a sample of healthy controls (CON, N = 46). SCH and MIX samples showed equal BCIS scores. The patient's Self-Certainty scores were comparable to the scores of the CON group. Regression analysis revealed Self-Certainty to be a significant positive predictor and Self-Reflectiveness to be a significant negative predictor of poor clinical insight. The psychological concept of cognitive insight may explain the difficulties of patients with schizophrenia to realize that they are mentally ill.