OBJECTIVES: To determine whether elderly people who meet national guidelines have higher physical function (PF) scores than those who do not and the effect on functional trajectory when physical activity (PA) levels change from above to below this threshold, or vice versa. DESIGN: Pooled data. SETTING: Two 6-month randomized controlled trials aimed at increasing PA in adults. PARTICIPANTS: Adults aged 65 to 94 (N=357). INTERVENTION: PA counseling over the telephone and through mailed materials. MEASUREMENTS: Self-reported PA dichotomized at 150 minutes/week and PF using the Medical Outcomes Study 36-item Short Form Questionnaire PF subscale. RESULTS: At baseline, individuals reporting 150 minutes or more of moderate PA/week had mean PF scores that were 20.3 points higher than those who did not (P<.001). Change in PA minutes from above threshold to below threshold or from below threshold to above threshold from baseline to 6 months resulted in an average change in PF of -11.18 (P<.001) and +5.10 (P=.05), respectively. CONCLUSION: These findings suggest that PA is an important predictor of functional status. Older sedentary adults can improve PF by meeting recommended PA levels. Conversely, dropping below recommended PA levels has a deleterious effect on PF. Given the importance of PF in maintenance of independence and quality of life in older adults, adherence to recommended PA guidelines should be endorsed. © 2008, Copyright the Authors.