The clinical significance of pulmonary function tests (including blood gas analysis) lies in their sensitivity for detecting PCP. PCP has most consistently been found to cause abnormalities in the DLCO and the exercise arterial blood gas; both are highly sensitive for the presence of Pneumocystis infection. These tests are more sensitive for the detection of PCP than are the resting arterial blood gas and chest x-ray. Therefore, measuring these values can be especially helpful in evaluating HIV-infected individuals who have pulmonary symptoms but whose resting arterial blood gas and/or chest radiograph are normal. The advantage of performing the exercise test over measuring the DLCO is that the exercise test is simple. It can be done without pulmonary function equipment and without a technologist. Furthermore, since many AIDS patients with non-PCP pulmonary disorders maintain "normal" exercise tests despite abnormal DLCO, it can be useful in evaluating patients for PCP who have known underlying lung disease with progressive symptoms. Measurement of lung volumes and spirometry lacks both sensitivity and specificity for detecting pulmonary disease in general and PCP in particular. Spirometry is helpful in detecting airways obstruction, which is not an uncommon finding in AIDS patients. The etiology, clinical significance, and treatment of obstructive ventilatory defects in the AIDS population remains unclear.