OBJECTIVES:    To develop a cross-cultural adaptation from Swedish into Spanish of the TooL questionnaire - a previously validated instrument to assess side effects’ impact on HRQoL in patients with schizophrenia and bipolar disorder. METHODS:   An epidemiological multicentre, cross-sectional study was carried out to analyze the psychometric properties of the Spanish TooL. It comprises 8 attributes and 4 levels per domain (Likert scale: 1-minimun impact; 4-maximun impact): worry-upset, function capabilities, fatigue-weakness, weight gain, stiffness-tremor, physical restlessness, sexual dysfunction, and dizziness-nausea. Patients completed both generic and specific measures of HRQoL and severity (EQ-5D and SF6D –unweighted- and the Clinical Global Impression -CGI-SI-, UKU side effects rating scale, Positive and Negative Syndrome scale, Young Mania Rating Scale, Montgomery Adsberg Depression Rating Scale). Reliability (Cronbach’s α and intraclass correlation coefficient –ICC-), construct validity (factorial analysis –FA- and item-total correlations -ITC), convergent validity (Spearman’s rank correlations between measures –rs-) and criterion validity (Mann-Whitney U differences between mild vs moderate-severe patients according to CGI-SI) were evaluated. RESULTS:    A total of 242 patients were included (121 with schizophrenia and 121 with bipolar disorder).  Internal consistency and ICC were adequate (Cronbach’s α= 0.757 & ICC= 0.90). FA and ITC showed that one-dimensional structure could be assumed (1 eigenvalue > 1 and 40 % of variance explained). Correlations (rs) between the Spanish TooL and both generic and specific measures were moderate-high.  Differences in the Spanish TooL scores between mild vs moderate-severe patients were highlighted. CONCLUSIONS: The Swedish TooL questionaire was culturally adapted and validated into Spanish. Further investigation is needed to test the sensitivity of the Spanish TooL questionnaire.