In September 2004, the National Institute for Occupational Safety and Health (NIOSH) received a management request for a health hazard evaluation (HHE) at Gro-West Inc. in Utica, New York. Gro-West management submitted the HHE request because they were remediating mold in a house being renovated for future use as a shelter for women and children. NIOSH was asked to evaluate the gaseous chlorine dioxide (ClO2) treatment process with respect to its effectiveness in removing microbial contamination. NIOSH investigators conducted an evaluation in November and December 2004. Traditional and newer techniques for evaluating microbial contamination were used under field conditions to evaluate the ClO2 treatment effectiveness. The evaluation was performed in a microbially contaminated house, which had an undetected roof leak for an extended period that resulted in large areas of visible microbial growth. Concentrations of culturable fungi and bacteria, total fungi determined by microscopic count and polymerase chain reaction (PCR) assays, endotoxin, and (1-->3)-B-D-glucan were determined before and after the house was treated with ClO2. Area air samples were collected and analyzed for volatile organic compounds (VOCs) present in the house before and after ClO2 treatment to see which VOCs were generated by the ClO2 treatment. Wipe samples of walls were collected for chloride, chlorate, and chlorite ion decontamination by-products before and after ClO2 treatment. Culturable bacteria and fungi concentrations and total fungal spore counts (as determined by spore trap and PCR) decreased significantly after the ClO2 treatment. However, microscopic analyses of tape samples collected from surfaces after treatment showed that fungal structures were still present on surfaces after ClO2 treatment. No significant differences in airborne endotoxin and (1-->3)-B-D-glucan concentrations were measured in the house before and after ClO2 treatment. An increase in chloride, chlorate, and chlorite ions occurred after ClO2 treatment, which was expected because these compounds are some of the end products of ClO2 disinfection. Due to the potential for health effects from residuals present after ClO2 treatment, additional clean-up techniques, such as using air cleaners and cleaning surfaces using high efficiency particulate air (HEPA) filter vacuums to reduce concentrations of spores and microbial components, were recommended.