Despite these well-meaning intentions, the practical clinical validity of VACs has not been established, and it is quite surprising that we have been asked to monitor for these events. For example, although not all VACs are VAP, we do not know how many cases of VAP are defined as VACs. In an early study of 600 patients in six US centers, Klompas et al3 found that 135 had VACs, whereas 55 had VAP. Similarly, an Australian study found that 153 of 543 patients had VACs, whereas only 40 patients with VACs had positive respiratory cultures and were treated with antibiotics,4 so that both studies found VACs to be more common than VAP. In contrast to these data are the findings of a Canadian study of 1,320 patients who were mechanically ventilated, of which the prevalence of VACs was 10.5%, whereas the prevalence of VAP was 11.2%.5 Of the patients with VAP, 79% had neither a VAC nor an IVAC. Similarly, Klein Klouwenberg et al6 found that the prevalence of VACs and the prevalence of VAP were similar in a group of 2,080 patients, but that the VAE algorithm identified only 32% of the patients with VAP. In the current issue of CHEST (see page 68), Boyer et al7 report their experience with a VAC in a prospective 1-year study of 1,209 patients who were mechanically ventilated. Their study had 67 VACs but 86 episodes of VAP, making VAP a more common event. In addition, most of the 86 episodes of VAP were not VACs, with only 15 probable cases of VAP and six possible cases of VAP being classified as IVACs.