This paper concerns a descriptive study of an uncommon disease characterized by abduction deformity of the shoulder secondary to fibrosis of the central portion of the deltoid muscle (acromio-humeral bridle—-AHB). The frequency of the condition in the city of Medellin, Colombia between 1960 and 1973 is suggestive of an epidemic outbreak. The proportion of cases in girls is higher than in boys. The median age of onset is 9 years. No cases have been identified in adults. The localization is 43 per cent both shoulders, 37 per cent left and 20 per cent right. The spatial distribution of cases by area in the city suggests that most of the cases occur in the lower middle socio-economic levels of the population. Aetiological hypotheses are discussed. An injection procedure used specifically for children (smallpox vaccination, DPT or some other drug) is the most attractive hypothesis.