Objectives To conduct a case–control study of abnormalities in the semen of genitourinary (GU) medicine clinic attendees compared with general practice (GP) controls and in patients with asymptomatic and symptomatic non-specific urethritis (NSU) before and after the urethritis resolves.

Methods Rates of semen abnormalities were compared between the different groups (19 with symptomatic and 27 with asymptomatic NSU, seven with symptomatic non-NSU and 64 clinic controls) and between clinic attendees and 417 patients attending GP for the first investigation of possible infertility. Those with symptomatic or asymptomatic NSU gave repeat semen samples on resolution of the NSU.

Results The study included 117 clinic volunteers. They were shown to have statistically significantly worse total sperm counts (p=0.002), volume of semen (p<0.001) and percentage of abnormal forms (p<0.04) compared with 417 GP controls. Compared with the rest of the clinic volunteers, asymptomatic NSU patients had statistically significantly lower total sperm counts (p<0.02). Asymptomatic NSU patients had statistically significantly lower total sperm counts compared with symptomatic NSU patients (p<0.02). Compared with GP controls, clinic controls had statistically significantly inferior total sperm counts (p=0.009) and semen volume (p<0.001).

Conclusions GU clinic attendees are more likely to have abnormalities of semen than patients attending GP for the first check for possible infertility. A high rate of abnormal semen findings are found in patients with and without NSU but the highest rate occurred in those with asymptomatic NSU. Is asymptomatic NSU therefore pathogenic and does it require treatment like symptomatic NSU?