Instructional forms of advance care planning depend on the ability of patients to predict their future treatment preferences. However, preferences may change with changes in patients' health states. We conducted in-home interviews of 226 older community-dwelling persons with advanced cancer, congestive heart failure, or chronic obstructive pulmonary disease at least every 4 months for up to 2 years. Patients were asked to rate whether treatment for their illness would be acceptable if it resulted in 1 of 4 health states. The likelihood of rating as acceptable a treatment resulting in mild (odds ratio [OR], 1.11; 95% confidence interval [CI], 1.06-1.16) or severe (OR, 1.06; 95% CI, 1.03-1.09) functional disability increased with each month of participation. Patients who experienced a decline in their ability to perform instrumental activities of daily living were more likely to rate as acceptable treatment resulting in mild (OR, 1.23; 95% CI, 1.08-1.40) or severe (OR, 1.23; 95% CI, 1.11-1.37) disability. Although the overall likelihood of rating treatment resulting in a state of pain as acceptable did not change over time (OR, 0.98; 95% CI, 0.96-1.01), patients who had moderate to severe pain were more likely to rate this treatment as acceptable (OR, 2.55; 95% CI, 1.56-4.19) than were those who did not have moderate to severe pain. For some patients, the acceptability of treatment resulting in certain diminished states of health increases with time, and increased acceptability is more likely among patients experiencing a decline in that same domain. These changes pose a challenge to advance care planning, which asks patients to predict their future treatment preferences.