OBJECTIVE To understand Ureaplasma urealyticum (Uu) infection, analyzed the influence of Uu infection on the seminal quality and the accessory genetical gland function in male infertility patients, and investigate its mechanism. METHODS We cultured 202 semen samples collected from male infertility patients and analyzed the influence of Uu infection on seminal parameters and the biochemical indexes of the seminal plasma. RESULTS The Uu infection rate was 33.7% in the infertile males, with no statistic differences between the Uu positive and negative groups either in the average age (28.9 +/- 4.7 yrs vs 29.6 +/- 4.0 yrs, P = 0.250) or in the seminal quantity (2.93 +/- 1.32 ml vs 2.86 +/- 1.52 ml, P = 0.774). The sperm density, motility and vitality were (84.37 +/- 52.92) x 10(6) ml, (44.62 +/-22.13) % and (38.40 +/- 15.61) % in the Uu positive group, significantly lower than (101.90 +/- 43.90) x 10(6) ml, (51.83 +/- 19.88) % and (44.45 +/- 15.47) % in the Uu negative group (P = 0.025, P = 0.036 and P = 0.020). The seminal pH value was normal in both of the groups, but significantly higher in the Uu positive than in the negative group (7.32 +/- 0.10 vs 7.19 +/- 0.29, P = 0.003). VCL, VSL, VAP and MAD were significantly lower, while BCF was significant higher in the former than in the latter [(33.97 +/- 8.96) microm/s vs (39.70 +/- 8.14) microm/s, t = 4.113, P < 0.001; (22.29 +/- 6.06) microm/s vs (25.20 +/- 6.67) microm/s, t = 2.684, P = 0.008; (25.96 +/- 6.83) microm/s vs (30.02 +/- 6.81) microm/s, t = 3.537, P < 0.001; 46.60 +/- 13.68 vs 54.23 +/- 15.14, t = 3.112, P = 0.002; (6.12 +/- 1.89) Hz vs (5.22 +/- 1.64) Hz, t = 3. 164, P = 0.002]. All the five indexes were influenced by Uu infection. Compared with the negative group, the seminal plasma alpha-glucosidase was significantly decreased in the positive group [(40.0 +/-18.7) U/ml vs (47.9 +/- 21.0) U/ml, t = 2.248, P = 0.026], and the risk of the decrease was 2.12 times higher. No statistic difference was observed in seminal plasma acid phosphatase and seminal plasma fructose between the two groups. CONCLUSIONS Uu infection in the genital tract is an important factor of seminal quality reduction in infertile men and may cause a decreased secretion of alpha-glucosidase in the epididymis, but it hardly influences the prostate and seminal vesicle.