Background and purpose: The trend towards a shorter stay in rehabilitation clinic has implications for future language therapy. Constraint-induced aphasia therapy (CIAT) is administered 3 h per day for a total of 30 h of treatment. It was evaluated for patients with chronic aphasia. In the present study we investigated the efficacy of a modified CIAT schedule and included patients with sub-acute stroke. We conducted a randomised, single-blind, parallel-group study. The results were compared to those of patients who received identically intensive treatment focusing on conventional aphasia therapy. Methods Fifty patients were treated with our modified version of CIAT and 50 received a standard aphasia therapy at the same intensity and duration. Inclusion criteria were clinical diagnosis of first-ever stroke, aphasia in the sub-acute stage and German speakers. Language function was evaluated using the Aachen Aphasia Test and the Communicative Activity Log directly before therapy onset, after the training period and at 8-week and 1-year follow-ups. Results Patients of both groups improved significantly in all sub-tests of the Aachen Aphasia Test Battery. The improvements remained stable over a 1-year follow-up period. Patients and relatives of both groups rated daily communication as significantly improved after therapy. Conclusions Both CIAT and conventional therapy performed with equal intensity are efficacious methods for patients with sub-acute aphasia. The modified CIAT schedule is practical in an everyday therapeutic setting. Our results indicate that a short-term intensive therapy schedule in the early aphasia stage leads to substantial improvements in language functions.