The distribution of drug acetylator phenotypes in 100 healthy newborn infants was studied and compared with the acetylator phenotypes frequency in different age groups. Phenotyping was performed by assaying total and free sulphadimidine in the urine after single oral test dose of the drug/100 mg. As in elderly subjects, slow acetylator phenotype was predominant also in healthy newborns (83%), which was the highest frequency of all age groups observed. Acetylator phenotype in the newborn infants may be influenced by genetic, environmental (e.g., nutritive), as well as by developmental factors (e.g., postnatal enzyme deficiency, age-specific changes in organ functions, etc.). In this connection, slow acetylator phenotype of the neonate may be related to a negative pantothenic acid balance causing insufficient Coenzyme-A synthesis, too. The predominance of the slow acetylator phenotype resulting in higher drug sensitivity can be regarded clinically as a special risk factor in human neonates.