Tests performed on urethral secretion and morning urine specimen identified the causative micro-organism in 143 of 164 males with non-gonococcal urethritis. In 59 patients (36%) there was an infection with Chlamydia trachomatis, in 48 (29.2%) with Ureaplasma urealyticum in significantly high bacterial counts (greater than or equal to 10(4) KbU/ml) urethral secretion (greater than or equal to 10(3) KbU/ml first urine sample), and in a further 18 patients (11%) double infections with Chlamydia trachomatis and Ureaplasma urealyticum. Other causative organisms such as Mycoplasma hominis, enterococci, streptococci of groups A and B, enterobacteria and Trichomonas were isolated in 18 patients (11%). In 21 patients (12.8%) no causative organism was demonstrated. In several instances, Chlamydia was demonstrated in ureaplasma-negative non-gonococcal urethritis, while Ureaplasma was demonstrated in Chlamydia-negative cases. In cases of urethritis caused by Chlamydia gonorrhoeal urethritis was more frequent in the past history than when urethritis was due to Ureaplasma. Treatment with tetracyclines led to clinically and microbiologically good results in 77% of patients with Ureaplasma urethritis, in 64% of patients with Chlamydia urethritis. Adnexial infections such as prostatitis and epididymitis were among the complications seen.