A cooperative trial of neutron therapy for soft tissue sarcomas was started in 1978. 112 unselected and not previously irradiated patients (62% T3 tumours, 35% recurrent tumours) were treated up to June 1982 with neutrons alone. An analysis of these cases with a mean follow-up period of 22 months (range 12 to 45 months) is given in this report. Very preliminary results are presented for 60 patients treated with a neutron boost only. These showed a substantially lower complication rate. The major results of this phase II trial are: The survival rate of 3.5 years was strongly dependent on the stage of the tumour; for 8 patients with T1 tumours it was 100%, for 35 patients with T2 tumours, 77%, and for 69 patients with T3 tumours, 45%. The survival rate at 3.5 years was strongly dependent on surgery before the beginning of radiotherapy; for 54 patients after surgery without clinical evidence of residual tumour it was 73%, for 58 patients with inoperable primary or recurrent tumour, 47%. The survival rate of 31 patients with recurrence after neutron therapy was only 36%. The overall rate of serious complications was 28.6% after neutron therapy, but only 5% after neutron boost therapy (mean follow-up period: 12 months, range 5 to 48 months).