Twenty-five patients with bacteriologically proven epididymitis were studied between 1984 and 1986. Thirteen were less than 35 years old and in 11 of them Chlamydia trachomatis was isolated; of the 2 remaining patients Escherichia coli was found in 1 and Neisseria gonorrhoeae in the other. In patients more than 35 years old, gram negative bacteria were the cause of infection in 10 and only 1 case was attributable to Chlamydia trachomatis. These results suggest that Chlamydia trachomatis is much more frequent in men under the age of 35 and the difference is statistically significant.