1. Resting pulmonary blood flow ($Q.$), using the uptake of the soluble inert gas Freon-22 and an indirect estimate of lung tissue volume, has been estimated during breath-holding ($Q.$c) and compared with direct Fick cardiac output ($Q.$f) in 16 patients with various cardiac disorders.

2. The effect of breath-hold time was investigated by comparing $Q.$c estimated using 6 and 10 s of breath-holding in 17 patients. Repeatability was assessed by duplicate measurements of $Q.$c in the patients and in six normal subjects.

3. $Q.$c tended to overestimate $Q.$f, the bias and error being 0.09 l/min and 0.59, respectively. The coefficient of repeatability for $Q.$c in the patients was 0.75 l/min and in the normal subjects was 0.66 1/min. For $Q.$f it was 0.72 l/min. There was no significant difference in $Q.$c measured at the two breath-hold times.

4. The technique is simple to perform, and provides a rapid estimate of $Q.$, monitoring acute and chronic changes in cardiac output in normal subjects and patients with cardiac disease.

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