In the past decade, human insulins have been substituting animal insulins, offering the advantage of its lesser antigenic capacity. One of the most clinically important problems with human NPH insulins is its tendency to flocculate. We present four diabetic patients who, after using flocculated human NPH insulin, encountered a deterioration in the metabolic control of their diabetes, and in two of them, there were bouts of diabetic Ketoacidosis "without any other apparent causal factors". Among those causes favoring flocculation are movement during transport, high temperatures, and probably leaving the vial open for an excessively long period of time, as with the extraction of multiple doses. Physicians, educators, diabetics, and their relatives should be informed of this phenomenon. Diabetics, especially those who carry insulin with them, should carefully inspect their vials before each injection to detect signs of flocculation.