To examine capillary permeability in fibromyalgia (FMS) we studied the nailfold capillaries of 13 unselected patients with FMS and 9 healthy controls using dynamic fluorescence videomicroscopy.The transcapillary permeability of a 20% solution of sodium fluorescein injected into an antecubital vein was assessed by videodensitometric analysis. The pericapillary and interstitial fluorescent light intensities (FLI) were calculated at different sites on a transverse axis crossing the selected capillary.Fluorescence videomicroscopy revealed no abnormalities in the pattern of dye distribution around the nailfold capillaries in the patient group. Videodensitometric analysis showed a trend to an increased early transcapillary diffusion in patients with FMS, but the difference was only significant at 3 s from the first appearance of the dye (p < 0.05). From 10 s to 1 min after the dye's appearance, the mean FLI in the patients equalled that of the controls. The patients with FMS showed an earlier but lower FLI peak. Moreover, the FLI were significantly lower in the patients than in the controls from 5 to 30 min after the dye appearance in all of the sites of the densitometric analysis (p < 0.05). Thirty min after the first appearance of the dye, the FLI was reduced by more than 50% in the patient group compared to the controls.Our results indicate that transcapillary permeability and the interstitial persistence of the tracer in FMS are significantly reduced compared to controls. This difference may be caused by the abnormal microvascular dynamics induced by low capillary flow and/or capillary bed hypotension.