The effect of patient positioning on thallium-201 images in the left lateral projection was evaluated in 28 patients. The left lateral image was performed with the patient on his right side (LLrs) and also supine (LLsup). False-positive inferoposterior defects were reported in five patients (18%) in the LLsup view, but not in the LLrs view. Image quality was better in the LLrs view. The false-positive results in the LLsup view may result from two factors: a) overlap of the left hemidiaphragm and myocardium; or b) changes in orientation of the heart in the two lateral positions. Therefore, thallium-201 images in the LL position should be performed with the patient lying on his right side.