Kinetics and clinical studies of bone scintiscanning with 99mTc-pyrophosphate in 92 patients with certified diagnosis allowed the following conclusions: 1. After i.v. injection 99mTc-pyrophosphate is distributed in 3 compartments, from which it is cleared with biological half-lives of 32.7 minutes, 5.1 hours and 22.54 hours respectively. The relative size of these compartments is 1 : 3 : 6. After application of the substance, an initially high renal excretion with a consecutive exponential drop can be observed. The optimal period for scintigraphic studies seems therefore to be from 3-6 hrs, as at this time there is still an adequate activity in bone structures, while the low background activity yields a good contrast on scintiscans. 2. The clinical results of bone scintigraphy with 99mTc-pyrophosphate with 88.6% correct diagnoses can be considered as "very good". This feature can partly be attributed to an improved correlation of radiographic and scintigraphic images by the use of a specially designed, combined radiographic-photoscanning detector system. 3. Preliminary results concerning the quantitative uptake of 99mTc-pyrophosphate in fractures of the extremities suggest, that the quantitative analyses of the registered data can provide valuable prognostic information in traumatic, inflammatory and neoplastic bone lesions.