Genetic selection of broiler chickens has led to a gross overdevelopment of the broiler breast muscle pectoralis major. This may have resulted in increased myopathy and detrimental effects on meat quality. The present study examined 3 commercial great-grandparent lines (lines A, B, and C). Lines A and B are female lines, and line C is a male line. The mean BW of line C (2.7 kg) was significantly greater than those of lines A and B (both 2.3 kg). However, the mean breast yield of both lines B and C (8.9 and 8.7%, respectively) was significantly greater than that of line A (6.9%). Line B therefore matched the meat yield of line A while maintaining a high reproductive capacity. The mean breast fillet weight of line A (169 g) was significantly lower than lines B (207 g) and C (235 g). No differences were observed between lines in either mean fiber size or amount of connective tissue. Therefore, additional fibers must provide the additional weight in the breast fillet of lines B and C, compared with A. Plasma creatine kinase activity, a commonly used marker of muscle damage, was significantly higher in line A (1368 IU/L) than in lines B (995 IU/L) and C (982 IU/L). However, qualitative evaluations of muscle pathology revealed no differences among lines. Selection for increased embryonic muscle fiber number, rather than for increased radial fiber growth, could improve growth potential and may also alleviate muscle damage.