To assess the frequency and quality of formal thought disorder in schizophrenic patients and their first-degree relatives, a consecutive series of 36 unmedicated patients, 20 siblings of these patients and 37 normal control subjects were examined with Holzman's Thought Disorder Index (TDI). As a proof of the internal validity of this tool, the patients demonstrated significantly more thought disorder than the controls as measured by the TDI total score and various subscores, which proved the internal validity of this tool in a German-speaking sample. In addition, in a pairwise comparison with controls who were individually matched by age and sex the patients' siblings had a significantly higher total TDI score and nonsignificant elevations on several subscores (combinatory and flippant). As to the level of severity of the thought disorder, the siblings mostly took an intermediate position between patients and controls. In conjunction with previous reports from other authors, our findings lend further support to the notion of subclinical thought disorder as an indicator of familial vulnerability to schizophrenia.