Thrombosis of the cerebral venous sinuses is characterized by clinical pleomorphism and variable pathogenesis. Although there is a relationship with pregnancy and especially with the puerperium, occurrence during the first trimester of pregnancy is unusual and this may effect the diagnosis and treatment. We describe the case of a 33 years-old patient with lateral sinus thrombosis in the first trimester of pregnancy. This was diagnosed and followed-up by magnetic resonance. Investigations as to its aetiology were inconclusive and it followed a benign course without anticoagulation. Thrombosis of the venous sinuses has been found empirically to be related to states of deficiency of S protein, hypercoagulability, anaemia and deshydration during pregnancy, to having previously taken contraceptive pills. Behçet's disease, other coagulopathies, central or systemic vasculitis, collagen disorders, septic and neoplastic disorders and a large number of cases of unknown aetiology. In our patient neurological deterioration secondary to a venous infarct or intraparenchymatous haemorrhage was not seen. There was full recovery and complete spontaneous canalization of the sinus, as shown on posterior cranial angiography. The controversial decision to use heparin in thrombosis of the cerebral venous sinuses should probably depend on the possible risks and benefits in each individual case, taking into account the clinical findings, aetiology and topography of the thrombosed sinus.