Transient tyrosinemia has been found in approximately 1.8% of all infants in Massachusetts tested within the first 6 weeks of life (2,085 infants out of a total screened population of 117,752). About 24% have been full-term by weight and gestational criteria. In general, infants with lower birth weights had more marked tyrosinemia. However, several babies with average or above average birth weights had blood tyrosine concentrations of 10 mg/100 ml to more than 20 mg/100 ml, and in a few of these latter infants the tyrosinemia lasted as long as 9 to 13 weeks after birth. The infants have been clinically well.