TQZ, LCB, GRR: Departments of Veterinary Diagnostic Medicine; WEM, VLK: Veterinary Clinical & Population Sciences; AFW: Veterinary PathoBiology, College of Veterinary Medicine, University of Minnesota, St Paul, Minnesota 55108 Summary: Analysis of litter size performance of 4388 boars used in single-boar matings revealed that 191(4.3%) were hypoprolific according to the criteria of Popescu. Boars classified as normal had sired litters with an average liveborn litter size of 9.984 ± 0.016 standard error (SE) piglets compared to 7.762 ± 0.057 SE (P < 0.0001) for the hypoprolific boars (HPB), a mean reduction of 2.22 piglets. Also, the average number of piglets born dead was significantly greater for the hypoprolific boars as compared to the normal boars (1.0236 ± 0.0309 SE versus 0.9138 ± 0.0085 SE; P = 0.0004). Based on our findings we propose an improved definition of hypoprolificacy based on that of Popescu.