Fluid collections following renal transplantation are not rare and may be associated with serious complications. We studied the incidence, clinical features, pathology and treatment outcome of perirenal fluid collections after kidney transplantation. Between January 1977 and June 1985, 386 consecutive renal transplants were performed at our university. All allografts were studied with B-mode ultrasonography together with a renal scan in the immediate post-transplant period, at 6-month intervals or when clinically indicated. Symptomatic fluid collections, those associated with rejection episodes and those containing more than 50 to 100 ml. fluid were aspirated under sonographic control via aseptic techniques. There were 190 fluid collections (49 per cent) observed during followup (2 to 11 years). Of these collections 98 (51 per cent) were estimated to be less than 50 ml. in volume, were clinically insignificant and resulted in no morbidity. A total of 92 collections was aspirated with 1 aspiration being diagnostic and therapeutic in 57 instances (serous or serosanguinous fluid). The 35 collections remaining were revealed to be lymphoceles on biochemical grounds. Of 13 lymphoceles associated with rejection episodes 8 resolved on initial aspiration. Of the recurrent lymph collections 27 were treated with repeated aspiration, tetracycline sclerotherapy or an operation (10 were treated with marsupialization into the peritoneal cavity). No large collections of urine or blood were detected and 1 infected lymphocele required external drainage. No renal allograft was lost as a result of a fluid collection and over-all graft survival was not affected by the development of perirenal fluid collections. We conclude that perirenal fluid collections are detected commonly in the post-transplant period using B-mode ultrasonography. The majority of these collections are small and will require careful observation only or they will resolve with a single aspiration. Aggressive diagnostic and therapeutic measures are used only for those collections that are symptomatic or result in allograft dysfunction. A rational approach to the diagnosis and treatment of peritransplant fluid collections is described in the form of an algorithm.