ABSTRACTPurpose:To investigate whether the semen quality of men undergoing conventional semen analysis is deteriorating over time.Materials and Methods:We analyzed and compared the sperm count, motility and morphology of 2300 semen samples provided by males undergoing conventional seminal analysis, from years 2000 to 2002 and 2010 to 2012. The incidences of severe oligozoospermia and azoospermia over time were also compared.Results:A total of 764 sperm samples were analyzed in 2000-2002 and 1536 in 20102012. Over time, the mean sperm concentration/ml decreased significantly from 61.7 million in 2000-2002 to 26.7 million in 2010-2012 (R2=11.4%, p<0.001), the total sperm concentration decreased significantly from 183.0 million to 82.8 million (R2=11.3%, p<0.001), and the percentage of normal forms decreased significantly from 4.6% to 2.7% (R2=9.8%, p<0.001). The incidence of severe oligozoospermia significantly increased from 15.7% to 30.3% (OR: 1.09, p<0.001) and the incidence of azoospermia increased from 4.9% to 8.5% (OR: 1.06, p=0.001).Conclusions:This study demonstrated a significant time-related decline in semen quality of infertile patients. This finding might have implications on fertility and emphasizes the need for further studies addressing subject's life-style in order to find and reduce the causative agents. Future prospective and multicenter studies including representative samples of the general population are needed to confirm whether semen quality is really declining.