Fluorescence microlymphography was performed near the medial ankle in 12 healthy controls, 12 patients with congenital and 12 with sporadic lymphedema of the lower extremities. Diameters of lymphatic capillaries were determined on the videoscreen by playback of the tape recordings using a morphometric technique. In the patients with congenital disease (Nonne-Milroy) aplasia of microlymphatics was diagnosed in 8 instances, ectasia in 4. Patients with sporadic lymphedema and manifestation after puberty exhibited initial lymphatics of normal caliber. In controls mean capillary diameter was 56.3 +/- 9.0 microns, in congenital disease with ectasia 99.3 +/- 9.0 microns (difference significant at the p less than 0.005 level) and in the sporadic form 49.7 +/- 7.7 microns. Congenital lymphedema may be subdivided into a form with aplastic and ectatic microvessels which possibly show different responses to therapy.