Prestressed concrete box beams are studied under combined loadings of torsion, bending, and shear. Thirty-seven eccentrically prestressed beams were tested—three under pure torsion, 10 under varying ratios of torsion and bending, and 24 under varying ratios of torsion, bending and shear. All beams had a 12-in. (305-mm) square cross section with an 8-1/2-in. (220-mm) diam hollow for the full length. Most of the beams contained web reinforcement in either of two stirrup spacings, but five beams without web reinforcement were also tested. Two shear span-to-depth ratios were utilized. Tests results include cracking patterns and strengths, ultimate failure modes and strengths, torque-twist, and flexural deformations. Analytical theories are developed for the initial cracking torque based upon elastic theory and for the ultimate strength based upon the skewed bending failure mechanism for prestressed concrete beams. Comparisons of the theoretical and experimental results are presented, and these generally correlated well.