OBJECTIVES: To identify bacterial species present in the lower genital tract of males and to investigate the relationship with semen quality. METHODS: The microscopic analyses and cultures of 696 semen specimens, collected over five years from males investigated for subfertility, were retrospectively assessed. RESULTS: Semen cultures were sterile in 48%; they showed a polymicrobial flora (more than two bacterial species) in 30%, and were positive (>1 × 10(3) colony forming units/ml) in 22% of the cases. Gardnerella vaginalis was the most frequently isolated bacterium, followed by Escherichia coli and Enterococcus sp. Ureaplasma urealyticum was recovered from 13 of 147 samples (9%). Of patients with bacteriospermia 42% had leukospermia (>10(6) leukocytes/ml of semen). Bacteriospermia and leukospermia did not correlate with each other although a positive correlation was found between the presence of leukocytes and G. vaginalis isolation. Semen parameters were correlated with the bacterial species isolated most frequently. In comparison with controls, sperm concentration, motility and morphology were mostly deteriorated in the presence of G. vaginalis and U. urealyticum. CONCLUSIONS: Positive seminal fluid cultures must be interpreted with caution, taking into account both raised colony counts of single isolates and leukocyte concentration in the semen. Thus the common misdiagnosis of genital tract infection, based on the presence of seminal bacteria, and unnecessary treatment with antibiotics may be avoided.