Riboflavin status indices in tissues (brain, liver, heart) and blood plasma, and performance parameters were studied in male and female broiler chickens in response to a wide range of dietary supplementation of riboflavin in order to establish the requirement for riboflavin in fast growing modern broilers. The birds fed riboflavin supplemented diets were increasing their body weight at a higher rate than those fed the unsupplemented diet, but this was apparent only during the first stage of growth (days 1 to 21). Supplementation of 2 mg riboflavin per kg was sufficient to support the maximum growth rate. Feed consumption was not affected by different levels of dietary supplementation of riboflavin. The supplementation of riboflavin in the diet increased (p < 0.001) plasma riboflavin level, but the magnitude of response decreased with age. The main component in the tissues was FAD, followed by FMN and riboflavin. Overall, the dietary riboflavin supplementation had highly significant (p < 0.001) effects on tissue FAD, FMN, and riboflavin status, but the effect of supplementation was clearly pronounced only at days 7 and 14, and thereafter the status of FAD, FMN, and riboflavin in the tissues did not differ between unsupplemented and supplemented birds. Neither FAD, FMN, and riboflavin nor GSSG-RED activity correlate with the level of supplementation. Saturation levels of riboflavin in the blood plasma and tissues, corresponded with dietary riboflavin levels of supplementation at 1 to 2 mg per kg. Based on the performance and biochemical data, the dietary requirement of riboflavin for fast growing broilers should be set at a level of 5 mg/kg. The currently recommended allowance of 3.6 mg riboflavin per kg of ration is not sufficient for modern breeds of broiler chickens.