Effects of feeding grain and maize silage from a non-Bt maize and a variety of Bt maize that contains cry1F (event TC1507, event DAS-Ø15Ø7-1), a gene that provides maize with insect resistance, on the health and performance of dairy cows were evaluated. In a crossover trial, 20 lactating Holstein cows were assigned to each of 2 dietary treatment groups and fed diets containing whole-plant maize silage plus maize grain from TC1507 or its near-isoline counterpart (control). Each period of the crossover trial lasted 28 d and was preceded by a 7-d adjustment period. To minimize variability due to stage of lactation, 2 blocks of 10 cows at 90 to 130 d of lactation at the start of the trial were used. Within each dietary treatment, 10 cows were from each of 2 genetic selection lines (high and average fat plus protein predicted transmitting ability). Diets were formulated to be isocaloric and isonitrogenous. Dry matter intake and daily production of milk, fat, protein, lactose, nonfat solids, and total solids did not differ between cows fed the TC1507 diet and cows fed the control diet. Furthermore, milk from cows in different dietary treatment groups did not differ in milk urea nitrogen concentration or somatic cell count. For milk fat percentage, a significant dietary treatment by genetic group interaction was detected although overall yield of milk and solids-corrected milk did not differ with diet. Physical measures of cow health including body weight, body condition score, temperature, pulse, and respiration rate were collected weekly; dietary treatment group means for these measures were not different. Blood chemistry and hematological analyses were conducted using blood samples collected from cows before the start of the trial and at the end of each period. Overall, the TC1507 and control groups did not differ in any of these indices of health status. Further, hematological profiles for cows in the dietary treatment groups were not different. In summary, no differences were detected in milk production, milk composition, or cow health as indicated by physical measures, blood chemistry, and hematological analyses between dairy cows fed diets containing maize grain plus whole-plant maize silage from TC1507 and dairy cows fed grain plus silage from its near-isoline counterpart.