Nineteen patients with leukaemia, preleukaemia, and aplastic anaemia were treated by marrow transplantation from HLA-identical siblings. All were given postgrading immunosuppression with a combination of methotrexate (days 1, 3, 6 and 11) and cyclosporin (days -1 to 180). In an attempt at reducing cyclosporin-associated toxicity, we explored whether the cyclosporin dose during the first 2 weeks could be decreased by 50% (from 3.0 to 1.5 mg/kg/d intravenously) without adversely affecting the incidence, onset, and severity of acute graft-versus-host disease (GVHD) and overall survival. Results from this pilot study were compared to those of a matched cohort of 38 patients given a standard dose of 3.0 mg cyclosporin/kg/d starting on day -1. The cumulative incidence of grade II and III acute GVHD in the 'low dose' cyclosporin group was 4.2% compared to 51% in the 'standard dose' group (P = 0.60). Three-year survival was 63% and 54% respectively (P = 0.59). Patients receiving the reduced cyclosporin dose during the first 14 d appeared to have less hepatotoxicity, and the methotrexate and cyclosporin doses administered were closer to the doses intended per protocol. We suggest that 'low dose' cyclosporin from day -1 to day 15 postgrafting might be as effective as 'standard dose' cyclosporin during that time period for the prevention of acute GVHD.