The environmental variables affecting Escherichia coli O157:H7 regrowth in dairy manure compost were investigated. Factors evaluated were moisture content, strain variation, growth medium of inoculum, level of background microflora and inoculum, different days of composting, and acclimation at room temperature. A mathematical model was applied to describe E. coli O157 regrowth potential in compost. Repopulation occurred in autoclaved compost with a moisture content as low as 20% (water activity of 0.986) in the presence of background microflora of 2.3 to 3.9 log CFU/g. The population of all three E. coli O157 strains increased from ca. 1 to 4.85 log CFU/g in autoclaved compost, with the highest increase in the spinach-outbreak strain. However, E. coli O157 regrowth was suppressed by background microflora at ca. 6.5 log CFU/g. By eliminating acclimation at room temperature and increasing the inoculum level to ca. 3 log CFU/g, E. coli O157:H7 could regrow in the presence of high levels of background microflora. E. coli O157:H7 regrowth in the autoclaved compost collected from the field study was evident at all sampling days, with the population increase ranging from 3.49 to 6.54 log CFU/g. The fate of E. coli O157:H7 in compost was well described by a Whiting and Cygnarowicz-Provost model, with R2 greater than 0.9. The level of background microflora was a significant factor for both growth and death parameters. Our results reveal that a small number of E. coli O157 cells can regrow in compost, and both background microflora and moisture content were major factors affecting E. coli O157:H7 growth.