Objective: To get insight into the changes over time of patients' involvement in the decision-making process, and into the factors contributing to patients' involvement and general practitioners' (GPs) communication related to the Medical Treatment Act (MTA) issues: information about treatment, other available treatments and side-effects; informed decision making; asking consent for treatment. Background: Societal developments have changed the doctor-patient relationship recently. Informed decision making has become a central topic. Patients' informed consent was legalized by the MTA (1995). Design: Data of two cross-sectional studies, the First (1987) and Second (2001) Dutch National Survey of General Practice, were compared. Setting and participants: General practice consultations; 16 GPs and 442 patients in 1987; 142 GPs and 2784 patients in 2001. Methods: Consultations were videotaped and rated using Roter's Interaction Analysis System and observer questionnaires; pre- and post-consultation patient questionnaires; and GP questionnaires. Descriptive analyses and multivariate, multilevel analysis were applied. Main results: Most patients reported to have received the information they had considered as important prior to the consultation. There were discrepancies in involvement in treatment decisions and in giving information about other available treatments, side-effects and risks. GPs who were more affective and gave more information, more often involved their patients, especially younger patients, in decision making. In 2001, more informed decision making was observed and the GPs asked consent for a treatment more often, but they less often asked for the patients' understanding. Conclusion: Patients' involvement in decision making has increased over time, but not in every respect. However, this does not apply for all patients, especially the older ones. It should be questioned whether they are willing or capable to be involved and if so, how they could be encouraged.