The assessment of social functioning takes on special importance with the development of promising new types of psychosocial intervention for the treatment of schizophrenia. Nevertheless, there are still few satisfactorily reliable instruments for the assessment of social functioning that are reasonably applicable in terms of time. This scarcity is even more notable in the Spanish language. The aim of this study is to validate a Spanish version of the Social Functioning Scale (SFS) giving new data (test-retest stability and concurrent validity) on the psychometric characteristics of the scale. One hundred and fifty subjects with a diagnosis of functional psychosis according to the DSM-III-R (codes 295-298) criteria - 105 (70%) of them schizophrenic patients - in an outpatient regimen of the Community Mental Health Center were assessed using a Spanish version of the Social Functioning Scale. The reliability and validity of the scale were studied. Data on the reliability (internal consistency, item-total correlation, test-retest and informant/self-report reliability) and validity (of the construct, concurrent and the criterion group method) revealed a substantial fulfillment of the basic psychometric criteria. The results corroborate the favorable psychometric characteristics of the scales and support the worth of the instrument for the assessment of the social functioning of psychotic patients when there is reliable and feasible assessment of out-patients.