The occurrence of urogenital involvements in female sexual partners of males with Reiter's syndrome (RS) or suspicion of RS (SRS) was studied. The possible etiological role of Chlamydia trachomatis (Ct) was demonstrated by isolation and by immunofluorescence (IF) serology. Evidence of chlamydial infection (positive isolation and/or IF titre greater than or equal to 64) was found in 35 out of 56 (62,5%) males with RS and in 9 out of 16 (56,3%) males with SRS. 43 female sexual partners of these men were studied. Evidence of present or past chlamydial infection was demonstrated in 23 of these 43 females (53,5%). This was a significantly higher frequency than that evidenced among controls studied, 14/77 verified serologically and 3/81 by isolation, p less than 0.0025 and p less than 0.005, respectively. A history of dysuria occurred in 10 out of 43 female sexual partners and in only 20 out of 364 interviewed randomly selected controls (p less than 0.00025). Abnormal urinary findings were also more frequent among the sexual partners than among the controls (p less than 0.025). The results emphasize the role of Ct as a triggering factor in RS and stress the importance of urogenital investigations among couples with RS.