OBJECTIVES: Pharmacotherapy is the most important component of care for patients with schizophrenia and related disorders. However, side effects adversely affect patient’s health-related quality of life . A novel self-assessment instrument (Tolerability and quality of Life, TooL) is presented to assess patient experiences of side effects. METHODS: The instrument was developed to reflect the subjective perception of side effects in patients treated with antipsychotic medication. TooL was developed by an expert panel of four experienced psychiatrists with a special interest in the treatment of schizophrenia, schizoaffective or bipolar syndrome. TooL has eight domains reflecting important aspects for patient well-being: anxiety and depression, usual activities, fatigue or weakness, body weight, stiffness and tremor, bodily restlessness, sexual function and vertigo or nausea. TooL was completed by 220 patients with schizophrenia, bipolar disorder or schizoaffective disorder in a cross-sectional observational study. Reliability and validity were estimated by correlations with existing scales assessing presence of side effects, symptom severity and HRQoL (UKU, PANSS, MADRS, SWN, EQ-5D, and VAS). RESULTS: Responses on TooL demonstrated that frequent experiences of side-effects impact the well-being of patients. Validity was supported by significant correlations with other instruments. TooL correlated with corresponding domains in the UKU scale. The MADRS score correlated strongest with anxiety, depression, fatigue or weakness. The global PANSS score significantly correlated with all domains except Weight gain, Stiffness and Sexual function. TooL also correlated to the VAS score, EQ-5D, and the disease-specific scale SWN. CONCLUSIONS: TooL is a new instrument for patient-reported assessment of the impact of side-effects of antipsychotic medication. The instrument can be of significant value in a clinical setting as a decision tool to guide treatment choices and serve as an important input to evaluations of antipsychotic treatment. Patient experiences of side effects are a highly relevant outcome measure that may determine the long-term success of therapy.