Venous thromboembolism (VTE) seems to be an underdiagnosed complication in COVID-19 patients. We present three male patients, aged 67, 29 and 71 years, who were admitted to the hospital with COVID-19. They all showed deterioration in the course of their disease caused by VTE. In our hospital, VTE was diagnosed in 10% of COVID-19 patients admitted to the general ward (non-ICU patients) despite regular thromboprophylaxis. Deterioration in the course of COVID-19 has differential diagnoses such as progression of the infection itself, secondary bacterial pneumonia, left heart failure and in our experience not infrequently VTE. We therefore recommend to consider VTE in COVID-19 patients with a sudden clinical deterioration such as hypotension, tachycardia, unexplained hypoxaemia or insufficient clinical improvement and to perform CT-angiography if indicated. A high dose of thromboprophylaxis in COVID-19 patients may be considered because of increased coagulation activation.