We report the results of two phase I1 trials of ifosfamide in very high risk patients with either partially responsive or recurrent non-Hodgkin's lymphomas. In the first study, in which patients were extremely heavily pretreated (50 per cent had received a very intensive salvage regimen containing very high dosc cyclophosphamide), there were two complete responses, two partial responses and one objective (minimal) response among 14 patients treated. Toxicity was acceptable even in this end-stage patient group. We concluded that ifosfamide is an active agent even in patients with tumours resistant to cyclophosphamide. The second trial was a pilot study in 13 patients of a regimen incorporating VP16, ifosfamide/mesna, and high dosc ara-C (VIPA). There were four complete responses, five partial responses and two objective responses. Two patients died in complete remission from toxic complications, while a third, with a stably regressed mediastinal mass died after completion of the protocol. While very toxic, we considered that this regimen was highly effective, and have since incorporated a slightly less intensive combination of the same drugs into the primary therapy of high risk patients. Since the primary toxicity of the VIPA combination was myelosupression, the use of a modified protocol incorporating colony stimulating factors to ameliorate the side-effects and possibly increase dose rate is worthy of further exploration in patients with recurrent B cell tumours.