We have applied sperm DNA typing to determine the distribution of crossover events within a one megabase region of the short arm of human chromosome 4 near the locus for Huntington disease. A total of 29 recombinants were detected among 602 sperm typed after whole genome amplification. These recombinants were typed for seven polymorphic markers. The 280 kilobase D4S10-D4S126 interval was found to undergo recombination at a 6-9-fold greater rate per unit of physical distance than the adjacent 720 kb D4S126-D4S127 interval. Sperm typing has the potential to dissect mammalian recombination hot spots to the point where DNA sequence analysis may reveal the molecular basis for hyperrecombination.