Mycobacteriosis caused by Mycobacterium avium subsp. avium was observed in a parental loft of 70 meat-breed pigeons. It was decided to undertake treatment as the birds represented a substantial value to the owner. A multiagent therapy using azithromycin, marbofloxacin, and ethambutol was administered. After 4 mo of therapy, the desired results were not obtained. At the end of treatment, the birds were in poor general condition, with white blood cells above 20 g/L, and after clutching, 2-yr-old and older birds were euthanatized. Overall, postmortem lesions were found in 17 out of 49 necropsied individuals. Slide agglutination tests with a M. avium subsp. avium lysate were conducted in all examined pigeons. In 28 pigeons, blood count was conducted once a month during therapy, while in 24 pigeons, a tuberculin sensitivity test was conducted before the planned euthanatization. The tuberculin sensitivity test did not prove useful in the diagnosis of ill individuals. Slide agglutination yielded positive results in only four birds, all of which also had postmortem lesions. Blood count in a large number of cases allowed distinguishing between ill and healthy individuals, which was used for subsequent selection. The comparison of cultured strains with the (CCG)4-based PCR method showed the variation of M. avium isolates up to a maximum of 30%. The described case proves that the treatment of mycobacteriosis in pigeon flocks is not effective, mainly due to the high resistance to M. avium subsp. avium. In addition, therapy may contribute to an even greater increase in mycobacterial resistance to antibiotics, which may pose a potential risk to public health.