Service load tests on the 6th floor of the full-scale reinforced concrete building at BRE Cardington are described. Both deflections and reinforcement strains were measured. Finite element analyses of the floor slab were then undertaken using the measured material properties and four different models for the behaviour of the concrete in tension, the tension stiffening effect being known to influence slab behaviour significantly. Calculated deflections and reinforcement strains from the four analyses were compared with values measured in the load tests. As a consequence, comments, reservations and recommendations are made concerning the use of FE analyses for predicting deflections and reinforcement strains. Finally, the need to appreciate and accommodate the indeterminate nature of many of the parameters involved is emphasized.