BACKGROUND: To study the effect of prolonged complex decongestive therapy (CDT) on lymphedema in arms without a subcutaneous echo-free space (SEFS) on subcutaneous tissue ultrasonography. METHODS: Fifty-one patients with arm lymphedema treated for longer than 1 year using CDT in our clinic were retrospectively evaluated. Before starting CDT, subcutaneous tissue ultrasonography was performed to examine for the presence of an SEFS. Two-stage CDT was performed as recommended by the International Society of Lymphology. Limb circumference was measured, and limb volume was calculated at the initial and latest visits. RESULTS: In patients with lymphedema in which SEFS was observed anywhere in the arm on the initial visit (n = 25), the edema ratio was significantly reduced by a median of -15% (range, -106% to 17%; P < 0.001). On the other hand, in the arms with lymphedema in which SEFS was not observed (SEFS[-], n = 26), the edema ratio was not changed significantly by CDT (median, 1% [range, -30% to 23%]). In arms without an SEFS that were not treated using arm sleeves regularly (n = 15), no increase in edema ratio was observed (median, 1% [range, -29% to 16%]). CONCLUSIONS: In arms with lymphedema without SEFS, the effect of CDT on the reduction of arm volume is limited.