In a retrospective study of 832 cases of thalidomide embryopathy (ThE) between October 1, 1959 and July 31, 1962, a highly significant accumulation of cases with infantile hypertrophic pyloric stenosis (IHPS) was registrated. Clinical course, X-ray and surgical findings and the sex ratio (male preponderance) were identical to IHPS occurring spontaneously. In the order of frequency of defects in ThE, IHPS is on position 11; among inner organ abnormalities, IHPS is on position 3 after heart and kidney defects. Thus, IHPS is the predominant gastrointestinal abnormality in ThE. For the first time, a substance (thalidomide) could be identified which obviously is able to influence manifestation of IHPS. There is a remarkable coincidence of IHPS and malformations such as hiatus hernia and tracheo-oesophageal fistula with or without oesophagus atresia. From the male preponderance, which is also observed in the ThE type of IHPS, it is concluded that thalidomide is not a primary cause in this process, but that disease manifestation is decisively influenced by thalidomide on the base of a genetic or familiar predisposition. Cause and development of IHPS are multifactorial; the mode of inheritance is polygenic. Probably, other substances may replace thalidomide.