Spontaneous tumour lysis syndrome (TLS) is an extremely uncommon cause for acute renal failure (ARF). ARF presenting with hyperkalemia, hyperuricemia and hyperphosphatemia should lead to further work up for occult haematological malignancy. We describe two cases of lymphoma presenting with ARF secondary to spontaneous TLS. The ARF in the first case appeared to be due to intravenous volume depletion but eventually lead to the diagnosis of gastric lymphoma. The second patient is interesting as he is probably the first reported case of lymphoma with HIV infection presenting as spontaneous TLS. Early diagnosis and prompt renal replacement therapy does not necessarily reverse the ARF.