Freezing and thawing sludge has become an economically feasible technique of sludge conditioning in regions where natural freezing is available. Even though the dramatic effects of freeze/thaw on dewatering have been studied extensively, its effects on the survival of pathogenic microorganisms have not been investigated. If freeze/thaw conditioning can be shown to reduce pathogenic microorganisms, then sludges treated in this fashion can be disposed of on land with less risk of health effects. In this paper, the effects of freezing rate, temperature and time in the frozen state on the removal of pathogens are tested. The response of seven indicators of microbial contamination; fecal coliforms, Salmonella, plaque forming units, fecal streptococci, poliovirus, helminths and protozoa to the freeze/thawing of both aerobically and anaerobically digested sludges is examined. Fecal coliforms, fecal streptococci, and plaque forming units are chosen as the commonly used indicators of bacteria and viruses, respectively in wastewater sludges. Pathogenic bacteria (Salmonella), virus (Poliovirus) and parasites (Ascaris suum and Cryptosporidium parvum) tested in the second phase are chosen as sample microorganisms in their respective classes. Freeze/thaw sludge conditioning is proven to be an effective means of removing most pathogenic microorganisms. Fecal streptococci are found to be the best indicator of the effectiveness of freeze/thawing on enteric bacteria. Results of this study indicate that freeze/thaw conditioning coupled with sludge digestion can significantly enhance the overall pathogenic microorganism reduction achieved in a wastewater treatment plant. Added effectiveness by freeze/thaw treatment may become critical for a plant in meeting the pathogen reduction requirements by U.S. regulations prior to any practice of land application or land disposal of sludge.