Norfloxacin (NOR) was given to 37 patients affected by urethritis due to Chlamydia trachomatis (CT), as demonstrated by clinical findings and fluorescent monoclonal antibody in urethral swab. The patients were divided into two groups according to a randomization list, and given either: NOR 400 mg t.i.d. or NOR 800 mg b.i.d. for 10 days. The 2 groups were comparable in terms of age, clinical presentation and duration of symptoms. Six out of 18 patients treated by regimen 2 had persistence of CT at the end of treatment (2 cases were clinically improved), while 4 out of 19 patients in group 1 dit not respond to the treatment. Overall 12 patients in group 2 (66.6%) and 15 patients in group 1 (79%) were asymptomatic and negative for fluorescent antibody at the end of the treatment. Both regimens were well tolerated. Our data show that NOR given at dosage higher than those usually recommended (e.g. 400 mg b.i.d.), may be of value for the treatment of urethritis due to Chlamydia trachomatis.