This study was carried out to investigate the trends in the reproductive performance of Holstein dairy cows in Iranian commercial dairy herds. The potential effect of year, season, herd, the number of parturitions, and their interaction on reproductive performance traits were investigated on 200644 records obtained from six herds from 2007 to 2018. A nonlinearly increasing trend in calving interval (CI; 414±1.7 to 418±0.74 days) and days open (DO; 132±1.48 to 136±0.58 days) were observed. Services per conception (SPC) increased from 1.94 ± 0.03 to 2.59 ± 0.01 in dairy cows but altered in a fluctuating manner in heifers. The number of days from calving to first service (DFS) was decreased (P<0.05) from 79 ± 0.54 to 66 ± 0.24 days. The conception rate (CR) declined in dairy cows (67±0.68 to 58±0.31; P<0.05%), but no difference was observed in heifers (P>0.05). A decreasing trend was also recorded in age at first service (AFS) and age at the first calving (AFC) of heifers over time. Herd, season, and the number of parturitions had a significant effect on reproductive traits. Reproductive performance was higher during winter and spring compared to summer and autumn (P<0.05). Cows with a number of parturitions less than four had the highest reproductive performance (P<0.05). A similar trend is presented in fertility parameters of Holstein dairy cows showing a reduction in reproductive performance in Iranian dairy herds. Although the exact reason(s) beyond these changes are not clear, it may be due to the increment of metabolic and environmental stressors which in turn disturb the biology of reproductive system. Our results are in line with the global decline in fertility of dairy cows and emphasize on need for adopting more efficient strategies to improve reproduction status in Iranian dairy cows, especially during summer and after third parturition. Manuscript profile