A perplexing issue in diagnosing the cause of renal allograft dysfunction is differentiation between rejections—the most common cause—and many other possibilities that have detrimental effects on graft function. This study was designed to determine whether technetium-99m sulfur colloid (TSC) accumulation could predict graft rejection. We prospectively studied 54 episodes of allograft dysfunction in 53 kidney transplant recipients who had undergone TSC scintiscanning and graft biopsy, within one week of evidence of allograft dysfunction. Visual analysis of TSC uptake was done by comparing allograft uptake with that of the fifth lumbar vertebra (L5) marrow. A 3+ result meant that allograft uptake was greater than L5 marrow uptake; 2+, allograft uptake was the same as L5 marrow uptake; 1+, less than and finally 0, no allograft uptake. Transplant accumulation of more or equal 2+ was considered consistent with rejection (P=0.01). Allotransplant biopsies were interpreted based on the Banff Working Classification and rejection was noted in 45 of 54 renal biopsies. 42 of 45 biopsy -proven rejection episodes had more or equal 2+ graft uptake. This nuclear medicine technique has a sensitivity of 93.3%, specificity of 44.4%, a positive predictive value of 89.3%, a negative value of 57.1% and an efficiency of 83.3% in the diagnosis of renal allograft rejection. In a retrospective analysis, among 500 recipients we found 101 patients (20.2%) with persistent elevation of hematocrit value (PTE). It was more frequent in males (82.2% and 17.8% in men and women, respectively). It occurred 2 to 50 months after engraftment (mean value was 11.2 ±8.9 months), majority of PTE which developed in the first 24 months (86%). Spontaneous remission of established PTE was observed in all cases within 3 to 93 months. PTE frequently occurred in patients with well-functioning renal graft, in 82.2% of cases the serum creatinine concentration was less than 1.5 mg/dl and it was 1.5-2 mg/dl in 15.8% of patients. There was no correlation between diabetes mellitus and PTE, compared with control group. PTE was more common in patients who received cyclosporine as compared to those who were not on cyclosporine. In conclusion, Post-transplantation erythrocytosis is a frequent problem in renal transplant patients. Predisposing factors include male gender, retention of native kidneys, cyclosporine consumption and rejection-free course with well-functioning renal graft. PTE is a self-limited complication but can result in thromboembolic disease.