Two elderly patients with prolymphocytic leukaemia (PLL) of T helper phenotype were treated with the adenosine deaminase inhibitor--deoxycoformycin--and achieved remission. The first patient has remained in an unmaintained remission for over a year. The second patient, treated with a regime which produced less side effects, subsequently relapsed in skin and lymph nodes and died. In view of the rarity of this condition a multi-centre assessment of the effectiveness of deoxycoformycin is indicated. T-lymphocyte colony formation in both cases was found to be reduced. Co-culture of the patients' lymphocytes with nonadherent mononuclear cells from normal individuals also showed inhibition of T-colony formation indicating that lack of nutrients or accessory cells was not responsible for low T-colony forming capacity.