Many Puerto Ricans classify illnesses, medicines, and foods according to an etiological and therapeutic system which derives historically from Hippocratic humoral theories of disease. Adherence to this system influences the way in which patients comply not only with therapeutic regimens for hypertension, arthritis, ulcers, and rheumatic fever but also with instructions on infant feeding, and antepartum and postpartum care. To achieve maximum therapeutic benefit for Puerto Rican patients who adhere to this system, the physician is advised to work within its framework in prescribing medicines and diet.