We evaluated a web-based intervention, the Healthy Weight Assistant (HWA), which was designed to help people with a healthy bodyweight, or those who are slightly overweight, to achieve and maintain a healthy weight. Four evaluation methods were used: (1) pre- and post-test questionnaires; (2) real time usability-tests; (3) log-file analysis; (4) qualitative analysis of forum posts, email messages and free-text responses in the questionnaires. A total of 703 respondents received access to the HWA. Six weeks after receiving access, 431 respondents completed a second questionnaire. The enthusiastic responses showed that many people were interested in using an interactive online application to support achieving and maintaining a healthy weight. The preliminary results suggest that improvements with respect to motivation may lead to large effects, yet require only small changes in the design of the HWA. Sending automatic tailored reminders may enhance motivation to keep using the application. Motivation to change behaviour may be enhanced by emphasizing goal setting and visualizing progress.