This case study examines the effect of an enclosure retrofit on a high-rise, multi-unit residential building (MURB). Literature on fan pressurization test methodologies and MURB air leakage rates is reviewed. The enclosure for the case study building was tested using the guarded-zone fan pressurization method. Results of the air leakage testing show significant improvement in the enclosure tightness and compare well to measured data for other MURBs across North America. There is recognition of a need to standardize both testing methods and presentation of data for air leakage in MURBs. The issue of abnormal flow exponent values is discussed and recommendations for future research are made.