This study was conducted to evaluate seasonal effects on production performance of lactation sows reared in two different environments in South Korea. A total of 76 sows (Landrace × Yorkshire) from the Dankook University experimental farm and 120 sows (Landrace × Yorkshire) from a commercial farm were collected. In the current study, reduced (p < 0.01) feed intake and energy intake during lactation were observed in warm season compared with cool season in both farms. Sows in cool season had higher backfat thickness at weaning but lower back fat loss (p < 0.01) than those in warm season. The weaning to estrus interval was lower (p < 0.01) in cool season than in warm season. Piglets weaned in warm season had lower (p < 0.01) body weight (BW), body weight gain (BWG), and average daily gain (ADG) at weaning than did those weaned in cool season on the Dankook University farm, whereas the reduction effect was only observed during 0 - 21 d on the commercial farm (p < 0.01). Moreover, piglets weaned in cool season had higher BW, BWG, and ADG (p < 0.05) on both farms. In conclusion, our results indicate that warm season had very negative effects on feed intake and production performance of lactating sows and piglets.