Combined positron emission tomographic (PET)/computed tomographic (CT) scanners allow the use of CT data for attenuation correction of PET images. Eight patients with cancer underwent PET/CT scanning. Transmission scanning was performed with conventional attenuation correction and with CT scanning during maximum inspiration and normal expiration. Image quality was visually compared and fluorine 18 activities were measured in volumes of interest in the lung and myocardium. Analysis of variance for repeated measures revealed a significant decrease (P =.0001) in measured activities between PET images corrected with CT data acquired during maximum inspiration and those corrected with the conventional attenuation correction method or with CT data acquired during normal expiration. Deep inspiration during CT can result in severe deterioration in the final PET image.