The article describes the possibilities of non-destructive testing of existing structures, made of autoclaved aerated concrete [AAC] masonry units. Relationships that allow the determination of the normalized compressive strength of autoclaved aerated concrete, were developed with the MDT semi-non-destructive method. The general form of the Neville curve, known from the diagnosis of plain concrete was used, which was calibrated to AAC of nominal density classes 400, 500, 600 and 700. The tests were performed and additional empirical relationships were built, to allow for the influence of density and moisture. Moreover, an ultrasonic non-destructive NDT method calibrated on standard cubic 100 × 100 × 100 mm samples of various densities and moisture, is presented. In addition to the non-destructive ultrasound method, the sclerometric method, calibrated on the entire masonry elements, limited to one nominal density of 600 kg/m3 with varying moisture, was also used. A practical application of AAC strength assessment in the existing masonry structure, has been presented.