In this issue, we evaluate seven models of disposable breathing circuits from seven manufacturers; the evaluated circuits are intended to be used for adult patients on critical care ventilators. This study is a follow-up to our earlier Evaluation of disposable breathing circuits for use with portable ventilators, published in Health Devices 22(7), July 1993. We based our ratings on the breathing circuits' ability to function properly with minimal intervention from the healthcare provider. We rated only one of the evaluated units Acceptable; this circuit was not likely to interrupt ventilation during normal use (provided that its operation was verified before use). Three of the evaluated breathing circuits were rated Unacceptable because they had disadvantages that were likely to interrupt ventilation during normal use (e.g., a component could accidentally disconnect, the tubing could become kinked or crushed). When such a disadvantage could be compensated for by the healthcare provider, we rated the circuit Conditionally Acceptable: one circuit was rated Conditionally Acceptable; two circuits that had other significant disadvantages were rated Conditionally Acceptable--Not Recommended. We caution readers not to base purchasing decisions on our ratings alone, but on a thorough understanding of the issues surrounding our conclusions, which can be gained by reading this study in its entirety. We offer additional guidance and discuss important selection factors and usage issues in the Selection and Use Guide for Disposable Breathing Circuits included at the end of the Evaluation. Also see "Pre-use Verification of Breathing Circuits" in this issue for step-by-step guidelines to help healthcare providers and caregivers verify proper performance and identify the defective circuits that inevitably reach the clinical setting.