An investigation was carried out to establish the incidence of X-linked progressive muscular dystrophies in Tuscany from 1966 to 1974. 52 cases were identified, the incidence being 23,95 X 10(-5) with reference to live-born males. The frequency of new mutations calculated according to Haldane's method was 7,97 X 10(-6). These data practically correspond to those reported by other AA. in Italy and abroad. There was also a remarkable variability in the incidence among individual districts with values ranging from 7,60 X 10(-5) to 41,91 X 10(-5). It was noticed that the number of new cases tended to decrease in the last few years.