It is not clear why some methamphetamine (MAMP) abusers develop psychotic symptoms, while others use MAMP regularly over long periods and remain unscathed. We tested the hypotheses that those users who develop MAMP-induced psychosis (MIP) have greater familial loading for psychotic disorders than users with no psychosis. Four hundred forty-five MAMP users were recruited from a psychiatric hospital and a detention center in Taipei, and were assessed with the Diagnostic Interview for genetic studies (DIGS-C) and the Family Interview for genetic study (FIGS-C). Morbid risk (MR) for psychiatric disorders in first-degree relatives was compared between those MAMP users with a lifetime diagnosis of MAMP psychosis and those without. The relatives of MAMP users with a lifetime diagnosis of MAMP psychosis had a significantly higher MR for schizophrenia (OR = 5.4, 95% CI: 2.0–14.7, P < 0.001) than the relatives of those probands who never became psychotic. Furthermore, the MR for schizophrenia in the relatives of the subjects with a prolonged MAMP psychosis (MIP-P) was higher than in the relatives of those users with a brief MAMP psychosis (MIP-B) (OR = 2.8, 95% CI: 1.0–8.0, P = 0.042). The greater his or her familial loading for schizophrenia, the more likely a MAMP user is to develop psychosis, and the longer that psychosis is likely to last. © 2005 Wiley-Liss, Inc.