Abstract. Amniocentesis for estimation of the lecithin: sphingomyelin (L/S) ratio was performed on 483 patients on 552 occasions. The introduction of this test has been associated with a highly significant (p less than 0-001) decrease of 59 per cent in the incidence of respiratory distress syndrome. The finding of a low L/S ratio (less than 2-0) enabled delivery to be deferred until ….