The present study was designed to determine whether formal thought disorder is less or more heritable than the global diagnosis of schizophrenia, and to examine relations among formal thought disorder and psychiatric diagnosis, stage of illness, and severity of psychiatric history. Interviews recorded in the Gottesman-Shields twin series (17 pairs of identical twins, 14 pairs of fraternal same-sex twins, and 12 unpaired twins) were assessed for the presence and severity of formal thought disorder, using a modified version of the Scale for the Assessment of Thought, Language, and Communication Disorders. No evidence of genetic influence on thought disorder was found. A factor analysis of the thought disorder scales revealed two interpretable factors reflecting verbosity and discontinuities in the form of speech. Scales with high loadings on the verbosity factor showed signs of familial influence and were significantly correlated with severity of psychiatric history. Scales with high loadings on the discontinuity factor were associated with the florid psychotic phase of illness.