The aim of this project was to study the kinetics of both Tl and Tc-99m MIBI in PBM by evaluating tumor to normal tissue ratio in early (E) images acquired within 1/2 hour and delayed (D) images acquired three hours following the i.v. injection of 3 mCi (111 MBq) of Tl and 20 mCi (740 MBq) of MIBI on 2 separate days in 49 patients. The washout index was calculated from E ratio minus D ratio divided by E ratio. A negative ratio indicating build up of activity in D images and a positive ratio indicated washout of activity from the E images. In addition, the findings were correlated with the following immunohistochemical parameters: pathological grading, number of cells in mitotic division (PCNA- Ki-67), angiogenesis (well formed and ill formed blood vessels) and presence or absence of Bcl 2 Oncogene (release antiapoptotic signals). Results showed that in all benign and malignant lesions, MIBI showed consistent washout varying from 19-27% while with Tl, there was persistent washout in all benign lesions and mixed washout or buildup varying from +16% to minus 17% in malignant lesions, (E) ratios showed a reasonable correlation between Tl and MIBI (r = 0.5). There was more significant correlation between the D ratios (r = 0.8). Due to high (E) MIBI uptake ratios and their higher percentage of washout than Tl, delayed ratios came close to each other. Immunohistochemical analysis revealed benign lesions presented with low mitotic rate: Ki-67 (71.4%), PCNA (14.2%), low amount of ill formed blood vessels (42.8%) and high amount well formed blood vessels (100%). While malignant lesions presented with high mitotic rate Ki-67 was (96.7%), PCNA (100%), high amount of ill formed blood vessels (73.3% in GII and 100% in Grade III) and less amount of well formed blood vessels of 90% and 83.4% in Grade II and III respectively. Bcl-2 was variable in both benign and malignant lesions with 71.4% in benign, 73.4% in GII and 16.7 in GIII malignancy. In conclusion, early uptake ratio in both benign and malignant tumors is related to the degree of angiogenesis, percentage of ill formed blood vessels, high mitotic activity reflected by high grade of tumor and high percentage of PCNA and Ki-67.