A means of measuring and monitoring skeletal growth rate using a plasma analyte, amino-terminal C-type natriuretic peptide (NT-CNP). Plasma NT-CNP concentrations reflect the potential for further growth of the immature skeleton. Measurement of plasma NT-CNP in a subject, when related to the mean of an appropriate age and gender matched set of control subjects, provides an index of the severity of a growth disorder not otherwise available and facilitates diagnosis in children with undetected osteo-chondrodysplasias or other intrinsic disorders of the growth plate. The invention also provides a means of detecting and monitoring potentially harmful effects of drugs and other factors on skeletal growth long before they are evident using current methods in clinical practice.