The objectives of this study were to investigate the associations of weaning-to-first-mating interval (WMI) groups with reproductive performance, to determine the repeatability and the correlation coefficients in WMI groups between consecutive parities, and to investigate factors associated with the proportion of sows having WMI 4-6 days. This study was conducted using 55,690 parity records of 11,991 sows born during 1999 in 94 herds. Five groups of WMI were formed: 0-3, 4-6, 7-20, 21-27, and > or =28 days. The correlation and the repeatability of the WMI groups were determined using correlation analysis and variance component analysis. Mixed-effects models were used to analyze the associations of WMI groups with reproductive performance, and the associations of parity, lactation length (LL), and nursing piglets with the proportion of sows having WMI 4-6 days. The overall proportion of the WMI 4-6 days was 82.3%. Sows with WMI 4-6 days had the highest farrowing rate, and had more pigs born alive than those with WMI 7-20 days (P<0.01). At each farrowed parity, sows with WMI 4-6 days had higher parity at removal than those with WMI 7-20 days (P<0.01). The repeatability of the WMI groups was low (0.08), and the correlation coefficients of WMI groups between consecutive parities were also low (0.10< or = r< or =0.18; P<0.01). More than 85.9% of sows with WMI 4-6 days were mated on 4-6 days postweaning at subsequent parity. Meanwhile, 65.8-83.9% of sows with WMI 0-3 or > or =7 days were also mated on 4-6 days postweaning at subsequent parity. Additionally, sows with parity > or =2, LL 24-28 days, and 9-10 nursing piglets were more likely to have WMI 4-6 days (P<0.01). In conclusion, sows having any WMI were more likely to be mated on 4-6 days postweaning at subsequent parity, and sows mated on 4-6 days had higher reproductive performance and higher longevity. Increased LL may increase the proportion of sows having WMI 4-6 days.