The occurrence of a left atrial thrombus without a haemodynamic predisposing factor (arrhythmia, mitral valvulopathy, severe left ventricular dysfunction) is a rare event. We report a case during the progression of refractory myeloma, four months after stopping treatment with thalidomide. The promoting haemodynamic factors for left atrial thrombosis in sinus rhythm, described in the literature, had been excluded. In our case the potential role of thalidomide is debatable, in the light of recent publications about venous and arterial thromboses observed with this treatment.