The aim of the present study was to show the distribution of cows by days in milk (DIM) at first AI, effect of DIM at first AI on reproductive performance and some factors influencing DIM at first AI. A total of 767 Holstein Frisian cows that calved from January 2004 to December 2007 in 14 dairy herds were used. The first AI conception rate (FAICR) was 34.0%. Seventy-five percent of the cows were first inseminated within 100 days after calving. FAICR increased linearly up to 100 DIM. A one unit (20 days) longer DIM at first AI within the first 100 days postpartum increased the likelihood of a 2.4% FAICR. However, cows first inseminated at an earlier stage of lactation showed better reproductive performance in terms of pregnancy rate and calving to conception interval than cows first inseminated at a later stage. A one day increase in the interval from calving to first AI reduced the likelihood of 0.85 days to become pregnant. Herd or region located in southwestern Japan, tie-stall herd, first AI in spring, higher parity, uterine infection, metabolic diseases and/or mastitis and abnormal resumption of postpartum ovarian cycles contributed to delaying first AI.