Objectives: The increasing prevalence of overweight and obesity suggests the use of the internet as a medium to provide individualised advice and counselling for a large number of interested people. Methods: www.lean-and-healthy.de is an interactive training-programme on healthy life-style and weight management. Based on the principles of behaviour modification, participants engage in weekly, interactive online sessions which guide and motivate changes of eating behaviour and physical activity over a one year period. These sessions are implemented as an automated dialogue with a computer system. In response to individual information provided by the participant through the input of questionnaires or diaries, participants receive individualised information, advice and training tasks. The computer-generated information is based on a psychological model of behavioural change. Sessions start with an evaluation of the last week's task assignment. After that participants get an individualised introduction into the topic of the new week followed by assignments for new behavioural training tasks, and finally an overall evaluation of the session is required. The data entered by the participants during theses sessions are used for the evaluation of the programme. Results: The programme has been launched in June 2001 for public use. Until August 2003 a total of 5167 active participants have registered (83.6 % women, 16.4 % men). Average baseline BMI was 29.1 in females and 30.8 in males. Over 30 % of the participants continue their participation for at least one year. An intention-to-treat analysis reveals that at least 13.3 % of initial participants achieve a weight reduction of 5 % or more. Conclusion: Computer-assisted individualised information delivered over the internet is a cost-effective approach to provide qualified advice and counselling to a large audience without specific restrictions with regard to time and place of participation.