In a carefully matched series of children suffering from myelomeningocele and normal controls, a significant increase was found in the levels of the tyrosine metabolites para-hydroxyphenylacetic acid, vanilmandelic acid and homovanilic acid,when these were expressed in terms of creatinine output. However, creatinine levels also showed a significant variation between patients and controls, being lower in the group of patients. It is suggested that the creatinine variations probably account for the whole of the variation in the other three constituents and that the common practice of expressing urine constituents on random specimens in terms of creatinine excretion be reconsidered, particularly in spina bifida. The reasons for the low levels of creatinine excretion are considered.