To evaluate the results of selective treatment of biopsy-proven mild acute rejection episodes, we retrospectively studied 1-week liver biopsies of 103 patients with a primary liver graft in relation to liver function tests. The overall incidence of rejection was 35%. In four patients the biopsy showed histological features consistent with rejection; in 27 patients it showed mild acute rejection (grade 1), and in 5 patients it showed moderate acute rejection (grade 2). Study group 1 consisted of 19 untreated patients with grade 1 rejection and group 2 of 8 treated patients with grade 1 rejection. At 30 and 90 days, no differences in liver function tests were found. The infection rate, histology after 1 year, and survival in the two groups did not differ. It may, therefore, be concluded that withholding treatment in histologically proven mild acute rejection is possible in selected patients based on histological, biochemical, and clinical criteria. This may reflect the functional diversity of morphologically similar lymphocytic infiltrates observed in graft biopsies showing features of mild acute rejection.