This paper describes the features of an outbreak of early syphilis in an institution for the care of adults with mental disorders. A case-note review was performed. In the period June to November 2001, 87 cases of syphilis were diagnosed [25 primary, 21 secondary and 41 early latent syphilis in 983 inmates (crude attack rate 8·9%)]. Among them 82 were heterosexual, four were homosexual or bisexual, and for one case sexual preference was not established. About half the cases were known to be promiscuous. The initial case was not identified. Penicillin therapy was administered to all cases and all known or suspected sexual contacts. Sporadic cases of syphilis have, however, continued to emerge from time to time. Institutions for patients with mental disorders are vulnerable to sexually transmitted diseases, and special strategies should be devised for their control.