OBJECTIVE Decompressive hemicraniectomy is an accepted treatment for otherwise untreatable intracranial hypertension. The aim of this prospective randomized study is to evaluate the benefit of application of collagen matrix as an onlay graft to reduce operating time during hemicraniectomy and to facilitate dural dissection during second-stage cranioplasty. METHODS Thirty-four consecutive patients were randomized to receive collagen matrix during hemicraniectomy or to undergo the conventional procedure. Specific time points were recorded during hemicraniectomy and cranioplasty. Intra- and postoperative complications, time course of Glasgow Coma Scale, Barthel's, and Early Rehabilitation Indices were monitored. The surgeon had to rate the convenience of the procedure if collagen matrix was used. Cost implications are discussed. RESULTS The use of collagen matrix during hemicraniectomy resulted in a reduction of combined operating time for hemicraniectomy and cranioplasty by an average of 19.7%. The rate of cerebrospinal fluid effusion during cranioplasty was 13% when collagen was used and 58% in the control group. None of the patients who received collagen developed cerebrospinal fluid effusion persisting longer than 1 week, compared with 33% of patients in the control group. A total of 85% of the surgeons rated the use of collagen matrix as being easier than usual; the rest did not see a difference. CONCLUSIONS The use of collagen matrix to cover the dural defect during hemicraniectomy reduces operating time in hemicraniectomy and cranioplasty. The complication rate (cerebrospinal fluid effusion), total treatment time, and time on intensive care unit can be reduced, giving a potential for cost reduction. There was no difference in the rehabilitative outcome.