The clinical relevance of symptomatic extension of spontaneous, acute, symptomatic, lower-limb superficial-vein thrombosis (SVT) is debated. We performed a post hoc analysis of a double-blind trial comparing fondaparinux with placebo. The main study outcome was SVT extension by day 77, whether to ≤ 3 cm or > 3 cm from the sapheno-femoral junction (SFJ). All events were objectively confirmed and validated by an adjudication committee. With placebo (n = 1500), symptomatic SVT extension to ≤ 3 cm or > 3 cm from the SFJ occurred in 54 (3.6%) and 56 (3.7%) patients, respectively, inducing comparable medical resource consumption (eg, anticoagulant drugs and SFJ ligation); subsequent deep-vein thrombosis or pulmonary embolism occurred in 9.3% (5/54) and 8.9% (5/56) of patients, respectively. Fondaparinux was associated with lower incidences of SVT extension to ≤ 3 cm (0.3%; 5/1502; P < .001) and > 3 cm (0.8%; 12/1502; P < .001) from the SFJ and reduced related use of medical resources; no subsequent deep-vein thrombosis or pulmonary embolism was observed in fondaparinux patients. Thus, symptomatic extensions are common SVT complications and, whether or not reaching the SFJ, are associated with a significant risk of venous thromboembolic complications and medical resource consumption, all reduced by fondaparinux.