Liu, J. B., Chen, D. W. and Adeola, O. 2014. Casein supplementation does not affect true phosphorus digestibility and endogenous phosphorus loss associated with soybean meal for broiler chickens determined by the regression method. Can. J. Anim. Sci. 94: 661-668. Cornstarch-based semi-purified diets impair growth performance of birds when using the regression method to estimate phosphorus (P) digestibility in assay feed ingredient. Therefore, casein is usually supplemented in the basal diet to improve dietary amino acid profile. The objective of this study was to investigate the influence of dietary casein supplementation on regression-derived estimates of true phosphorus digestibility and endogenous phosphorus loss (EPL) associated with soybean meal (SBM) in broiler chickens. Four hundred thirty-two male Ross 708 broilers were allocated to nine diets in a 3×3 factorial arrangement of three diet types (with casein at moderate dietary P, without casein at moderate dietary P, or without casein at low dietary P) and three SBM levels (low, medium, or high). There were six birds per cage and eight replicate cages per diet in a randomized complete block design. The birds were fed experimental diets from day 12 to 20 post-hatching. Chromic oxide was used as an indigestible marker to calculate P digestibility and retention. The results indicated that body weight gain and feed efficiency linearly increased (P<0.01) and dry matter retention was linearly decreased (P<0.01) with the addition of SBM, regardless of diet type. The apparent prececal P digestibility and excreta P retention were increased by casein supplementation (P<0.01). The prececal and excreta P output linearly increased (P<0.01) with graded inclusion of SBM in each diet type. Regression of P output against dietary P intake showed that the type of diets (with casein at moderate dietary P, without casein at moderate dietary P, or without casein at low dietary P) had no effect on true prececal P digestibility and excreta P retention, or EPL associated with SBM for broiler chickens (P>0.05). In conclusion, the results of the current study indicate that adding casein, which is high in P digestibility, to cornstarch-based basal diets does not affect the estimation of EPL or true P digestibility and retention associated with SBM for broiler chickens using the regression method.