Although numerous sarcolemmal protein defects in muscular dystrophies have been identified, the mechanisms linking these defects and muscle fibre degeneration are not fully characterized. As there is evidence that apoptosis is part of muscle fibre loss in dystrophin-deficient mdx-mice, apoptotic muscle fibre death may also play a role in humans with muscular dystrophies. We investigated in-situ DNA-fragmentation by the TUNEL-method and expression of apoptosis-related proteins immunohistochemically in 14 children suffering from deficiencies of dystrophin, adhalin, and merosin, and found TUNEL-positive chromatin-cleavage of muscle fibre nuclei in about 10% of non-necrotic muscle fibres. DNA-fragmentation also occurred in groups of 'necrotic and regenerating' muscle fibres with labelling of nuclei in myogenic cells and phagocytizing macrophages. These lesions also revealed expression of apoptosis-promoting factors, such as bax and ICE, inducing cleavage of myofilaments, and of the apoptosis-inhibiting proteins bcl-XL and bcl-2 which neutralized high bax levels. Mimicking embryonal myogenesis, chromatin-fragmentation in 'necrotic and regenerating' areas seems to be part of the regulating events in muscle regeneration to eliminate excessive proliferating satellite cells. Nevertheless, macrophages are also affected by apoptosis after successful removal of necrotic fibres. In humans, DNA-fragmentation and expression of apoptosis-related proteins indicate that apoptosis plays a role in muscle degeneration and regeneration in muscular dystrophies.