Synopsis For an accurate assessment of the service load response of a concrete structure, the tension stiffening effect of concrete between major cracks needs to be taken into account. Previous studies of tension stiffening have been mainly concerned with solid beams and slabs. At present there is no generally accepted method for determining tension stiffening in voided slabs. This paper presents the results of tests on fifteen 1: 3·33 scale model transverse strips of practical reinforced concrete voided slabs. A method of predicting tension stiffening in transverse strips of voided slabs is presented, and a simplified procedure suitable for design purposes is proposed.