Postthrombotic syndrome (PTS) may affect 50% of patients with deep venous thrombosis, 5–10% of them may present severe manifestations. The causes for PTS development and severity have not been well established. This study evaluated whether PTS may be associated with the presence, and echogenicity, of the residual vein thrombosis (RVT). We included patients with a history of deep venous thrombosis in the past 58 months. These patients were further evaluated for PTS diagnosis, clinical comorbidities, plasma levels of D-dimer, serum levels of C-reactive protein and for the presence of RVT. Particularly, RVT was detected by ultrasound examination and the residual thrombi echogenicity was determined by grayscale median (GSM). Fifty-six patients were included, of which 41 presented PTS. Mild PTS was detected in 23 patients, moderate PTS in 11 and severe PTS in seven patients. Patients with severe PTS showed higher body mass index, higher abdominal circumference and higher C-reactive protein levels when compared with the other patients (P = 0.007, P = 0.002, P = 0.02, respectively). The ultrasound-generated GSM was significantly lower in patients with severe PTS compared with patients with mild–moderate PTS or no PTS (median = 24, 35 and 41, respectively; P = 0.04). A GSM value less than 25, which was consistent with a hypoechoic RVT, was the best cut-off value to discriminate patients with severe PTS from those with mild or moderate PTS and those without PTS. RVT is a common finding among patients with PTS and the echogenicity of the RVT may impact the severity of PTS.