We aimed to evaluate the possible effects of seasonal variation on semen parameters. We retrospectively analysed the data of 6,116 semen samples collected at a university hospital for eight years. The past ambient temperature, relative humidity and daylight duration records, and birth registry of the province were obtained to examine the relationship of seasonal changes in semen parameters with annual birth rates and environmental factors. The mean age was 33.03ʱÊ6.86Êyears. We found a significant difference between months for sperm concentration (pÊ<Ê.0001), total sperm count (pÊ<Ê.0001), progressively motile sperm count (pÊ<Ê.0001) and normal sperm morphology (pÊ=Ê.028). The sperm concentration and total count were significantly lower in July and August compared with December, May and June. The progressively motile sperm count in October was 23.6% less than the value of May. The temperature and temperature-humidity index were negatively correlated with semen parameters. The highest number of births was in the summer. However, no correlation was present between deliveries and the semen concentration regarding months (r(s) Ê=Ê0.199, pÊ=Ê.083). In conclusion, we observed significant seasonal and monthly differences in sperm concentration, sperm count and progressively motile sperm count. Increased ambient temperature due to seasonal changes may be a detrimental factor for semen parameters.