—We are grateful for the opportunity to reply to Dr Levinson's comments on our article, and we welcome his interest. However, we disagree with his comment that there is a "reasonable possibility of a misdiagnosed affective psychosis in the schizophrenic twin." The Maudsley twin subjects used in our study are quite elderly and are therefore certainly not "young patients with psychotic affective disorder" who might be assigned a misdiagnosis of schizophrenia. Although the original diagnosis was made before the advent of operational definitions, a panel of six diagnosticians from different theoretical orientations made the consensus diagnosis of schizophrenia.1 More recently,2 we have applied operational definitions to the same series, and, if there was doubt about the schizophrenia diagnosis, these patients were excluded (for instance, the MZ probands numbered 10A and 12A). Thus, all the remaining probands fulfilled at least one set of modern operational criteria for schizophrenia