This study provides new knowledge about the factor structure of the 12-item General Health Questionnaire (GHQ-12; D. Goldberg, 1972) through the application of confirmatory factor analysis to longitudinal data, thereby enabling investigation of the factor structure, its invariance across time, and the rank-order stability of the factors. Two community-based longitudinal adult samples with 1-year (n = 640) and 6-year (n = 330) follow-up times were studied. As a result, the correlated 3-factor model (i.e., Anxiety/Depression, Social Dysfunction, and Loss of Confidence) showed a better fit with both samples than the alternative models. The correlated 3-factor structure was also relatively invariant across time in both samples, indicating that the scale has good construct validity. The rank-order stabilities of the factors were low across time, which suggests that the GHQ-12 measures temporal mental state.