Side effects of foam sclerotherapy for varicose veins can include both deep (DVT) and superficial leg vein thrombosis (SVT). The risk factors that favor the development of SVT or DVT after foam sclerotherapy are still largely unclear. The aim of our retrospective analysis was to use a larger group of patients with thromboembolic complications to identify both patient-related and procedure-related risk factors for thromboembolic complications from foam sclerotherapy. A total of 170 patients who received foam sclerotherapy were examined. With reference to a cut-off date, March 17th, 2020, the 85 most recent patients with thromboembolic complications (study group A) were included and compared to the most recent 85 patients without thromboembolic complications (control group B), after sclerotherapy with foamed sclerosant. Patients with a thromboembolic complication were more likely to have thrombophilia (11/85 vs. 3/85). The mean BMI values in group A (25.9 ± 5.1) were significantly lower than in group B (28.0 ± 7.2) (P = 0.034). Thromboembolic complications were more likely to appear after foam sclerotherapy on the lower leg (61/105) than on the thigh (1/13) (P < 0.001), particularly after dorsal than after ventral foam sclerotherapy (39 of 47 vs. 5 of 40, P < 0.001). Of the 39 thromboembolic complications on the dorsal lower leg, 23 were muscle vein thromboses. The risk of muscle vein thrombosis after foam sclerotherapy is especially increased in slender patients with sclerosed, dorsal lower legs.