24 perirenal masses in renal allograft recipients were aspirated by means of ultrasonographically guided percutaneous fine needle puncture, analysed biochemically, bacteriologically and identified as 15 lymphoceles, 5 hematomas, 2 abscesses and 2 urinomas. The discrimination criteria, based on macroscopic findings and biochemical analysis, are previously described in detail. According to ultrasonographic and clinical characteristics the lymphoceles are divided into two groups: 1. Large lymphoceles without any sign of rejection, which seem to be caused by injury of lymphatic vessels in the pelvic region of the graft recipient. 2. Small lymphoceles with signs of rejection, which evidently result from increased secretion of lymph by the allograft. In 54 punctures a therapeutic effect was achieved in small abscesses and lymphoceles only, which were drained percutaneously. A repeated fine needle evacuation of lymphoceles did not result in an obliteration of the lymphocele in any case. Ultrasonographically guided fine needle puncture of perirenal masses is of great importance, since the perirenal fluid can be identified exactly and an immediate and definitive therapy is made possible.