The objective of this study was to determine the inevitable endogenous amino acid (AA) loss at the terminal ileum of broilers that were fed diets with 2 different fiber levels using a regression approach. The design of the study was a randomized complete block employing a factorial arrangement of treatments with 3 CP levels (50, 90, and 130 g/kg) and 2 fiber levels. The fiber level was adjusted by inclusion of cellulose at the expense of cornstarch. The AA pattern of the CP was the same in all diets. Titanium dioxide was used as indigestible marker. Six cages of 8 birds were allocated to each diet. The experimental diets were offered for ad libitum consumption for 3 d, starting on 21 d of age. Digesta were sampled on a cage basis from the distal two-thirds of the intestine section between Meckel's diverticulum and 2 cm anterior to the ileo-ceca-colonic junction. Inevitable endogenous CP and AA losses were determined by extrapolating the linear regressions between intake and prececal flow toward zero intake. The inevitable losses of CP and AA, expressed in relation to DM intake, were significantly increased by increased cellulose inclusion in the diet. Amino acids with the greatest loss were Glu, Asp, and Thr, whereas Met was the AA with the lowest loss. The ranking of the concentrations of AA of inevitable CP loss was very similar between the 2 fiber levels. This ranking also was similar in comparison to published values for the endogenous AA losses in broilers. It was concluded that the fiber level in the diet can affect the amount of AA inevitably lost at the terminal ileum and that all AA are affected to a similar extent. The results suggest that there is no effect of enhanced fiber level in the diet on AA composition of prececal endogenous CP loss in broilers. These findings can be considered in modeling the AA requirements of broilers.