Two soy-based infant formulas, Neo-Mull-Soy and Cho-Free, were taken off the market in 1979 when they were found to be deficient in chloride. Some children born between 1971 and 1979 who used these formulas have been found to have persistent expressive language disorders, particularly word retrieval problems, as well as other developmental problems. Medical studies of such children are reported, and a case study describes and contrasts the development of fraternal twins, only one of whom had the chloride-deficient formula. Although both twins were normal at birth and developed at the same rate before Neo-Mull-Soy was prescribed for one of them, their history diverges sharply from that point. Language and learning ability test results and academic achievement are compared for the language disabled and the normal twin. Children with similar problems may come to the attention of speech-language pathologists throughout the United States.