Genetic improvement in the raising of broiler chickens contributes to the increase in effectiveness of this agricultural production branch. At the same time, a new phenomenon is reported: the occurrence of numerous metabolic disorders and diseases of the bone and muscle systems. A particular example thereof is the anomaly of pectoral muscles in broiler chickens referred to as deep pectoral myopathy (DPM). The objective of the paper was to analyze the issue of myopathy of musculus pectoralis minor including the identification of DPM symptoms, frequency, causes, and possibility to limit this anomaly. Changes typical of developing DPM are, initially: ecchymoses, edemas, pink colouring of m. pectoralis minor, and, during subsequent phases, green and greenish-grey muscle colouring with symptoms of necrosis. The occurrence frequency of myopathy depends on many factors. It was found that in Poland the incidence rate in large populations of broiler chickens ranged from 0.15% to 0.9%, and, also, in some cases, to 1.9%. A similar intensity was also recorded in several EU countries. In the USA, the frequency of incidence was estimated to be 0.7% on average, although some cases are known with a rate ranging from 3 to as much as 17%. There are considerable economic losses owing to the utilisation of m. pectoralis minor as well as to the reduced value in use by consumers and the reduced technological usefulness of m. pectoralis major. The experimental forced and controlled wing flapping in fast-growing commercial Ross and Hubbard Flex genetic lines caused the DPM symptoms to occur. Based on the risk analysis of DPM occurrence, it was possible to determine that the flock selection, disturbance in the flock hierarchy (unsynchronized removal of birds from the same facility/birds are picked up from the same facility more than once), noise, and sounds near the rearing unit are factors to cause the highest risk in the chicken production on the Polish commercial poultry farms.