Outbreaks of the asymmetric hindquarter syndrome (AHQS) in two pig herds are reported together with the results of test matings and dissection studies on affected pigs. The condition (apparently identical with that described in Germany in 1967), is characterised by a variable asymmetry of the hindquarters and particularly of individual muscles of the posterior, lateral and medial aspects of the thigh. It does not noticeably interfere with locomotion but can adversely affect carcase conformation and may detract from the usefulness of carcase dissection studies as a basis for genetic selection. AHQS has a familial incidence and is usually first detected clinically after weaning. A variety of degenerative and dystrophic changes have been seen in the muscles of AHQS pigs but these are not constantly present in nor confined to 'undersized' muscles. The precise aetiology of AHQS is still unresolved but it is concluded that it probably occurs post-natally and that some pigs have a genetic liability to develop the condition.