X-ray microdensitometry was performed on small wood samples after fungal exposure to assess wood density loss and its variability within a hybrid larch population. Microdensitometry was applied to thin wood slices obtained from increment cores collected from standing trees before and after 4 weeks of exposure to Coniophora puteana fungus. Density loss rates were measured and compared to mass loss percentages calculated on wood blocks following a 16-week decay test according to CEN TS 15083-1 (2005). The relationship between the standard mass loss and the density loss percentages was weak. Several explanations for the results are presented in this work, and methodological improvements are suggested to achieve a more accurate comparison. The proposed screening test has several advantages over the current standard method, as it is less invasive for the tree, less time consuming, and is therefore better suited for genetic studies and breeding. As predicted, due to extractive content variation and fungus behaviour, density loss after 4 w was more important in earlywood than in latewood and in inner heartwood than in outer heartwood. The new method also better determined the differences in decay among trees within the larch population than the standard method at 16 w.