1. Two experiments were carried out to investigate responses in performance and bone compositional and structural characteristics in broilers fed diets containing 4 concentrations of vitamin D₃ (5, 20, 125 and 250 µg cholecalciferol/kg) at different concentrations of calcium, available phosphorus and vitamin A.

2. In experiment 1, body weight and tibia breaking strength were maximised at 14 d with 250 µg vitamin D₃/kg, tibia ash was maximised with 125 µg vitamin D₃/kg. A high incidence of tibial dyschondroplasia (TD) was decreased to very low levels with 125 µg vitamin D/kg.

3. At 42 d, performance and bone characteristics showed no response to vitamin D₃ concentrations above 20 µg/kg.

4. Dietary vitamin A within the range 2·4 to 4·5 mg retinol/kg did not show any interaction with vitamin D₃ status at either age.

5. In experiment 2, responses to vitamin D₃ were strongly influenced by dietary calcium/available phosphorus. With 13 g calcium and 5 g available phosphorus/kg, performance and bone characteristics responded to vitamin D₃ concentrations up to 125 µg/kg but more was needed at less optimal concentrations of calcium and available phosphorus. TD incidence was minimised with 250 µg/kg.

6. This study shows that high dietary concentrations of vitamin D₃ can prevent TD. It is concluded that the vitamin D₃ requirement of broilers up to 14 d of age at optimal dietary calcium and available phosphorus concentrations may be in the range 35 to 50 µg/kg for cortical bone quality and up to 250 µg/kg for prevention of TD. The vitamin D₃ requirement for cortical bone quality after 14 d is not higher than 20 µg/kg. These requirements are much higher than earlier estimates and may be related to higher calcium requirements of modern broiler genotypes. Current regulations limiting maximum vitamin D₃ concentrations in broiler starter diets may need to be reviewed.