Taurine deficiency occurs in a large number of cats fed unfortified commercial diets. Deficiency arises because cats are unable to absorb all the taurine in processed diets and/or are unable to synthesize the deficit between absorption and requirement, which makes taurine an essential amino acid for cats. Taurine-depleted cats develop retinal degeneration, cardiomyopathy, altered white-cell function, and abnormal growth and development. Taurine deficiency is best estimated from the plasma-taurine concentration, with values less than 30 mumol/l considered deficient.