Dietary patterns reflecting food habits may be associated with chronic diseases, yet little is known about the stability of these patterns. The objective of this study was to observe over time the stability of dietary patterns measured with exploratory and confirmatory factor analysis. Four random subsamples of 1000 women between 49 and 70 y old were chosen from >60,000 women included in the Swedish Mammography Cohort. Subjects in these subsamples were administered a FFQ 4, 5, 6, or 7 y after the baseline questionnaire; 3607 of the women responded (90% response rate). The stability of dietary patterns was evaluated with Spearman correlation coefficients between pattern scores at baseline and follow-ups and by a test of internal stability, which evaluated the significance of changes within patterns between baseline and follow-up. We found 3 major dietary patterns: a healthy pattern, a Western pattern, and an alcohol pattern. Correlations between explored dietary pattern scores at baseline and at follow-up decreased from 0.59 (P < 0.01) after 4 y to 0.50 (P < 0.01) after 7 y for the healthy pattern, from 0.47 (P < 0.01) to 0.39 (P < 0.01) for the Western pattern and from 0.54 (P < 0.01) to 0.46 (P < 0.01) for the alcohol pattern. After 4 and 5 y, there was no evidence for internal instability in any of the 3 patterns. The Western pattern became internally unstable after 6 and 7 y and the alcohol pattern was unstable after 7 y. Our findings for this specific population suggest that in longitudinal studies, dietary exposures should be updated after at least 7 y.