More than 3500 inherited metabolic diseases are known to exist and their rates of morbidity and mortality vary enormously. Identification of these patients, usually neonates or young children, is important for appropriate treatment, prognosis, and genetic counselling. Patients suspected of having an inherited metabolic disease should be investigated for abnormal elevation of metabolites in the urine and blood. Laboratory techniques for the determination of endogenous substances in body fluids are remarkably diversified, ranging from simple chemical tests to sophisticated chromatographic analysis coupled with information-rich detectors. Definite diagnosis of most disorders requires identification of the deficient enzyme or aberrant transport proteins. Analysis may be carried out directly on urine, plasma, or blood cells, or in cultured fibroblasts or lymphocytes requiring cell culture facilities. For many inherited metabolic diseases, recombinant DNA technology has been proven reliable in the detection of affected patients and disease carriers. DNA analysis has also elucidated the molecular genetics and pattern of inheritance involved.