Fifty patients from the outpatient clinics of the University Hospitals of Cleveland were followed up with multiple urine tests for isoniazid (INH) and aminosalicylic acid (PAS) during a six-month period to discover the extent of patient cooperation. Fifteen (30%) of 50 patients receiving isoniazid and 14 (42%) of 33 patients receiving aminosalicylic acid were uncooperative. Isoniazid and aminosalicylic acid were equally acceptable to patients who were taking both drugs; however, isoniazid was more frequently omitted by those taking both drugs than by those who took isoniazid alone. The study demonstrates the pitfalls of relying on clinic attendance, prescription pick-up data, and response to therapy in assessing patient cooperation. Because of the experience of this study and that reported in the literature, it is urged that those who evaluate drugs determine whether patients take the drug in question and restrict their study groups to those who do.