Leg weakness, lameness and other bone abnormalities connected with various metabolic disorders are continuing problems in rapidly growing meat-type chickens, leading to considerable production losses and having a negative effect on bird welfare (Julian, 2005;Waldenstedt, 2006;Dibner et al., 2007). It has been reported that the bones in modern broiler lines are characterized by poor calcification and high porosity, which may cause an increased affinity for bone damage (Williams et al., 2000). The reduced walking ability caused by bone disorders can lead to difficulties in feed intake and a decreased body weight for chickens in production. The causes of bone abnormalities in broilers are usually complex, having a substantial genetic component and a high correlation with the growth rate. However, nutrition can also have an effect on the development of bone disorders and thus its optimization may be a strategy for decreasing the severity of leg lesions in broilers. Vitamin D 3 is one of the main nutritional factors crucial to Ca and P absorption and proper skeletal development. It is generally added to diets in the form of cholecalciferol; however, in order to carry out its physiological function, it must be hydroxylated in a two-step process: in the liver (to 25-OH-D 3 ) and in the kidneys (to 1.25-OH-D 3 ). Whitehead et al. ABSTRACT: A 6-week experiment with broilers was conducted to study the effect of diet supplementation with organic acids on performance, characteristics of the tibia and femur bones, and the calcium, phosphorus and zinc balance. A total of 320, one-day-old, Ross 308 chickens were randomly assigned to 1 of 8 treatments. A 2 × 4 factorial arrangement was used, with two dietary levels of calcium and available phosphorus (standard -9.4/9.2 g Ca/kg and 4.3/4.0 g available P/kg, or reduced -8.3/8.1 g Ca/kg and 3.7/3.5 g available P/kg, for the starter/finisher feeding phases, respectively), and with diets supplemented with organic acids (none; short chain fatty acids (SCFA), 4.0 g/kg; medium chain fatty acid (MCFA), 2.0 g/kg or SCFA + MCFA, 3.0 + 2.0 g/kg). Broilers fed diets supplemented with SCFA or MCFA displayed a performance similar to those fed the unsupplemented diet (P > 0.05). At 42 days, reducing the dietary levels of Ca and P decreased such bone parameters as tibia yielding load (256 vs. 270 N) and tibia stiffness (171 vs. 184 N/mm), as well as femur breaking strength (342 vs. 369 N), yielding load (233 vs. 250 N), stiffness (164 vs. 174 N/mm), and cortex thickness (1.47 vs. 1.56 mm). The organic acids had no effect on the parameters of the tibias; however, SCFA and SCFA+MCFA increased the yielding load and stiffness of the femurs. The SCFA diet supplementation significantly increased the relative retention of Ca (45.0 vs. 41.1%). There were no significant Ca and P level × organic acids interaction effects on performance parameters, bone quality indices or the Ca, P, and Zn balance. It was thus concluded that SCFA can improve the bone quality and Ca balance in broiler chickens when fed eit...