The objective of this study was to quantify the associations between summer climate data and reproductive performance of sows in lactation by herd productivity groups and by parity in Japanese commercial herds. This study was conducted by using 56,779 parity records of 45,506 sows farrowed between June and September in 2007 and 2008 in 113 herds. The climate data included records of daily maximum temperatures and daily relative humidity reported in 21 weather stations of Japan Meteorological Agency. Average values of the maximum temperatures (maximum temperatures) and relative humidity (relative humidity) for 21 days from the farrowing for individual sows were coordinated with reproductive data of the sows. The 113 herds were classified into high-, intermediate- and low-performing herds on the basis of pigs weaned per mated female per year. Parity records of sows at farrowing were classified into two groups:1 and 2 or higher. Two-level analysis was applied using a herd at level 2 and an individual record at level 1. Means (±SEM) of maximum temperatures and relative humidity during summer from 2007 to 2008 were 28.1±0.01°C and 74.2±0.02%, respectively. Increased maximum temperatures during summer were associated with prolonged weaning-to-first-mating interval (WMI), lower farrowing percentage and fewer subsequent pigs born alive (subsequent PBA; P<0.05). Increased relative humidity were associated with prolonged WMI (P<0.05), but not with farrowing percentage and subsequent PBA (P0.05). There were two two-way interactions between maximum temperatures and herd productivity groups or parity for WMI (P<0.05). As maximum temperatures increased by 1°C, WMI of sows in high-performing herds increased by 0.08 days, whereas those in low-performing herds increased by 0.21 days. As maximum temperatures increased by 1°C, WMI in parity 1 and 2 or higher sows increased by 0.16 days and 0.10 days, respectively. A two-way interaction between relative humidity and parity was found (P<0.05). In parity 1 sows, WMI increased by 0.10 days as relative humidity increased by 1%, whereas WMI in parity 2 or higher sows did not increase. A two-way interaction between the maximum temperatures and parity was found for farrowing percentage (P<0.05). A parity 1 sow lactating at 30°C of the maximum temperatures was 0.95 times as likely to farrow as a sow lactating at 25°C. Meanwhile, a parity 2 or higher sow lactating at 30°C was 0.98 times as likely to farrow as a sow lactating at 25°C. There was a two-way interaction between maximum temperatures and herd productivity groups for subsequent PBA (P<0.05). As maximum temperatures increased by 1°C, subsequent PBA of sows in high-performing herds decreased by 0.06 pigs, whereas subsequent PBA of sows in low-performing herds decreased by 0.09 pigs. In conclusion, this study indicates that negative effects of high temperatures during lactation on WMI and subsequent PBA are different between herd productivity groups. Additionally, it is harder in parity 1 sows than parity 2 or higher sows that negative effects of high temperatures or relative humidity during lactation on WMI and farrowing percentage.