For the first time measurements of lymph flow velocities in cutaneous microlymphatics of patients with lymphedema were performed and compared with healthy subjects. Flow velocity in single lymphatic skin capillaries was measured using fluorescence video microscopy after subepidermal microinjection of FITC-dextran 150,000 in 15 healthy volunteers and 16 patients with primary lymphedema. Initial filling of the lymphatic capillary network was fast with significantly higher mean velocities in patients with primary lymphedema than in healthy controls (890 +/- 43 vs. 550 +/- 390 microns/s, p < 0.05). The resting velocities were not significantly different between controls and patients (10.3 +/- 4.1 vs. 16.6 +/- 13.9 microns). In 12 out of the 16 lymphedema patients cutaneous backflow of the fluorescent contrast medium from deeper invisible lymphatics was observed. In 4 of these patients rhythmic reflux with a mean frequency of 1.4 +/- 0.5 cycles/min was measured by video densitometry in microlymphatics with a significantly (p < 0.01) enhanced diameter. Mean flow velocity (Vp) in these precollectors was significantly increased compared to the resting velocities (p < 0.01). On the basis of these results the hypothesis is advanced that rhythmic cutaneous backflow originates from intrinsic contractions of deeper lymph collector segments and is transmitted to the superficial microlymphatics through incompetent connecting channels. This newly recognized mechanism appears to be an important factor for the pathophysiology of lymphedema.