The dimensionality of the Hospital Anxiety and Depression Scale (HADS) is a current source of controversy among experts. The present study integrates a solid theoretical framework (Clark & Watson's, 1991, tripartite theory) and a fine-grained methodological approach (structural equation modeling; SEM) to examine the dimensionality and construct validity of the HADS in fibromyalgia (FM) patients. Using the HADS data of 269 Spanish patients with FM, we estimated the cross-sectional and, for the first time, longitudinal fit (autoregressive model) of 2 competing models (oblique 2-factor vs. bifactor) via confirmatory factor analysis. The pattern of relationships between the HADS latent dimensions and positive and negative affect (PA and NA) was analyzed using SEM. HADS reliability was assessed by computing the omega and omega hierarchical coefficients. The bifactor model, which accounted for the covariance among HADS items with regard to 1 general factor (psychological distress) and 2 specific factors (depression and anxiety), described the HADS structure better than the original oblique 2-factor model during both study periods. All latent dimensions of the bifactor model were temporally stable. The SEM analysis revealed a significant link between psychological distress and NA as well as between depression and low PA. Only the general factor of psychological distress showed adequate reliability. In conclusion, the HADS shows a clear bifactor structure among FM patients. Our results indicate that it is not recommendable to compute anxiety and depression scores separately because anxiety variance is tapped primarily by the broader construct of psychological distress, and both specific dimensions show low reliability.