To analyze the situations that make chemoprophylaxis for tuberculosis difficult. One hundred twenty-eight patients consecutive (106 HIV negative and 22 HIV positive) diagnosed of tuberculosis (TB) were studied. The patients were interviewed and a questionnaire was filled out in order to identify risk groups and determine what steps had been taken to prevent TB. In the HIV negative group, 63 (57.8%) had at least one risk factor. The most common were contact with persons with active TB (31.1%), former TB (15.1%), rapid weight loss or chronic malnutrition (13.2%) and residence in closed institutions (5.6%). Of the 51 (48.1%) for whom evaluation of chemoprophylaxis was indicated, 43 (84.3%) had been examined by a physician within the past five years; only 10 (23.3%) of them, however, had been checked for TB and isoniazide had been prescribed for only 4 (9.3%). In the HIV positive group, 13 (72.2%) of those for whom evaluation of chemoprophylaxis was indicated had been seen by a physician; 12 (97.7%) of them were given tuberculin tests checked for TB and isoniazide was prescribed for 4 (30.7%). None of the patients in either group who were prescribed a full course of prophylaxis actually took the drug enough. Most HIV negative patients for whom evaluation of chemoprophylaxis was indicated had been examined by a physician in the five years before disease was detected; less than a quarter of them were checked for TB, however. This situation is probably a consequence of the structure of health care in Spain as it affects TB control. Nearly all the HIV positive patients were checked for the disease, as they benefited from protocolized health care.