99mTc-mercaptoacetyltriglycine (MAG3) is a new renal radiopharmaceutical which has been shown to have biological properties similar to 131I-hippurate (OIH) in animals and volunteers. 99mTc-MAG3 has now been compared with 131I-orthoiodohippurate (OIH) in a group of patients with varying degrees of renal impairment. In all cases, the 99mTc-MAG3 images were superior regardless of serum creatinine. Selected examples are illustrated including scans and renogram curves in a normal volunteer, transplant patients with creatinines of 9.8 mg/dl and 2.6 mg/dl respectively, and a furosemide study in a patient with questionable obstruction. Our preliminary results suggest that 99mTc-MAG3 performs well in patients with impaired renal function and may well provide an acceptable replacement for OIH. A kit formulation has been developed and will soon be undergoing clinical evaluation.