Chlamydia trachomatis was isolated from rectal swab specimens taken from six of 18 homosexual men with presumed lymphogranuloma venereum (LGV) proctitis. All 18 men met standard serologic criteria for the diagnosis of LGV (complement-fixing antibody titer, greater than 1:16; microimmunofluorescent IgG antibody titer, greater than 1:1,000). However, four of the six chlamydial isolates were non-LGV strains of C. trachomatis. It is suggested that these strains may cause proctitis and result in the exuberant antibody response associated with LGV. Thus, serologic confirmation of LGV proctitis may be "false-positive" in the sense that it reflects other chlamydial infections.