During the past 30 years there has been a steady growth in the size and number of ships that use the Strait of Istanbul (Bosporus) which is one of the most hazardous, crowded, difficult and potentially dangerous waterways in the world. There have been over 200 accidents over the past decade resulting in loss of life and serious damage to the environment. This paper presents the results of a real-time ship manoeuvring simulation study investigating the manoeuvring performance of large tankers in the Bosporus. The study was conducted with a ship manoeuvring simulator which is capable of subjecting a given hull form to any combination of environmental conditions, i.e. wind, current and wave drift forces. The results indicate that when realistic environmental conditions are taken into account the size of ships which can navigate safely in compliance with the traffic separation lanes is limited. (C) 2003 Elsevier Science Ltd. All rights reserved.