Clinical investigations have been made in 70 patients to determine whether 169Yb-labelled metal ligand complexes accumulate in various tumors as was shown by previous animal experiments. The time course of these radiopharmaceuticals in tumors, the skeleton and the liver permitted a good to excellent tumor imaging in 61/70 cases. Even 2-5 h after intravenous injection of the radioactive compound, a meaningful diagnosis could be made in the majority of cases. A number of lesions were found for which no equivalent findings were available with other methods. They were therefore interpreted as metastases.