Determinants of prices of 1,302 weanling Thoroughbreds sold at the 2010 November Breeding Stock Sale at the Keeneland Association, Inc. are investigated. Weanlings, who are less than one-year-old, are of unknown breeding and racing quality; still, prices at auctions display high variability. A hedonic price analysis is utilized to identify the determinants of weanling sales prices, and the corresponding marginal values of those determinants are estimated. Prices were highly responsive to variables related to pedigree quality, including the sire’s stud fee, whether the sire was relatively new in his breeding career or not, and whether the dam or the dam’s progeny had earned black type. In addition, prices were influenced by individual characteristics such as gender, age, state of birth. The weanling’s placement in the sale and pinhooked weanlings also influenced price. Results can be used as a decision tool by both buyers and sellers.