The clinical pulmonary infection score-original or modified-has been proposed for the diagnosis and management of ventilator-associated pneumonia. In 79 episodes of suspected pneumonia, we prospectively assessed the diagnostic accuracy of the physicians' clinical assessment of probability and of the modified clinical pulmonary infection score, both measured before (pretest) and after (post-test) incorporating gram stains results, using bronchoalveolar lavage fluid culture as the reference test. The pretest clinical estimate was inaccurate (sensitivity 50%, specificity 58%); the mean clinical pulmonary infection score at baseline was 6.5 +/- 1.3 (range, 3-9) and 5.9 +/- 1.7 (range, 3-9), respectively, for the 40 confirmed and the 39 nonconfirmed episodes (p = 0.07), and only slightly more accurate (sensitivity 60%, specificity 59%) than the clinical prediction. Incorporating the gram stain results of either directed or blind protected sampling increased the diagnostic accuracy (sensitivity and specificity of 85% and 49% and 78% and 56%, respectively) of the clinical score and increased the likelihood ratio for pneumonia of a score of more than six from 1.46 to 1.67 and 1.77. The clinical pulmonary infection score has low diagnostic accuracy; however, incorporating gram stains results into the score may help clinical decision making in patients with clinically suspected pneumonia.