The aim of the present study was to use data from herds to demonstrate the degree of seasonal influence on litter size at birth in gilts compared to sow parities 2, 3–5 and older (parities ⩾6) in a conventional, open-housing system for commercial pig herds in the northeastern part of Thailand. Data were obtained during a 3-year period from July 2005 to June 2008. The data analysed included observations on 25 835 litters from 8100 sows. Total number of piglets born per litter (TB), number of piglets born alive per litter (BA), proportion of stillborn piglets per litter (SB) and proportion of mummified fetuses per litter (MF) were analysed using a general linear mixed model procedure. The influence of temperature, relative humidity and temperature-humidity index (THI) on TB, BA, MF and SB were also analysed. The meteorological data were merged with the reproductive data and the means of temperature, relative humidity and THI during 115 days before farrowing were calculated and included in the statistical models. The results revealed that sows that farrowed in the hot season had a larger TB and BA than sows that farrowed in the rainy (P<0·001) and cool seasons (P<0·001). The difference of TB and BA among seasons was more pronounced in the gilt litters than the sow litters, insofar as the gilts that farrowed in the rainy season had 0·7 TB fewer than gilts that farrowed in the hot season (P<0·001). By contrast, sows of parities 2, 3–5 and ⩾6 that farrowed in the rainy season had 0·4 (P=0·01), 0·3 (P=0·003) and 0·3 (P=0·02) TB fewer than those that farrowed in the hot season. In the first parity, MF increased from 0·022 to 0·042 when the mean temperature during gestation increased from 26 to 29°C (P<0·001). On average, a reduction of 0·8 TB and 0·7 BA were found when the humidity during gestation increased from 50 to 80% (P<0·001). The influence of THI on TB, BA and SB differed among herds. When THI increased from 71/72 to ⩾81, a decrease of 0·4 TB were observed in herd A (P<0·001) and a decrease of 0·9 TB were observed in herd B (P<0·001), but not in herds C and D (P>0·05). In conclusion, inferior litter size at birth was observed in sows that farrowed in either rainy or cool seasons. High temperature, high relative humidity and/or high THI during gestation significantly reduced the number of total piglets born per litter. The influence of season, temperature, relative humidity and/or THI on litter size at birth was more evident in the gilts than the sows. These data indicated that various strategies to reduce temperature in the open-housing system for pregnant gilts and sows in Thailand are not adequate and the proper housing of pregnant gilts should be emphasized.