The actual worldwide incidence of congenital hypothyroidism (CH) is based on the results of screening in parts of the world where screening is mandatory, i.e. most of Europe, USA, Canada, Cuba, Australia, New Zealand and Japan. In other parts of the world, some indications are given by the results of pilot studies. In Europe, mean overall incidence (1985-1990) for the countries included in our inquiry is 1/3801 – in each country for the same period: Austria 1/3,930, Belgium 1/3,750, Czechoslovakia 1/6,037, Denmark 1/3,777, Finland 1/3,969, France 1/4,132, FRG 1/3,827, Greece 1/3,314, Hungary 1/5,632, Israel 1/3,152, Italy 1/3,150, The Netherlands 1/3,723, Norway 1/3,069, Portugal 1/3,139, Spain 1/3,216, Switzerland 1/3,913, UK 1/3,398 and Turkey (pilot study 1989-1992) 1/2,943. In comparison, the figures for the USA for the whole country are similar for the mean overall incidence (1988-1990): 1/4,119. But large variations exist between the states, the reasons of which are perhaps related either to technical problems or to the ethnic background in each state. In Canada and Japan, modifications of screening procedures have led to similar figures for the last years in our possess, Canada (1986-1988) 1/3,884 and Japan (1990) 1/3,856. The figures (mean overall incidence) for Cuba (1987-1991) 1/2,325, for Australia (1985-1990) 1/1/3,331 and for New Zealand (1987-1990) 1/4,496 are quite comparable. So in these countries where the screening is established, no great variations are noted as in certain parts of the world with pilot studies: Argentine (Buenos Aires; 1985-1990) 1/4,407, Chile (1991-1992) 1/2,514, Brazil (Porto Alegre; 1987-1991) 1/4,429. In Mexico (1990-1991), it is 1/1,480; however, the overall incidence is higher, in the two pilot studies carried out in Ecuador. On the other hand, a pilot study carried out in São Paulo (1982-1991) on 1,794,051 tests is 1/14,352; the low incidence should be related to the ethnic background or/and techniques of screening.