Frontal lobe lesions may present as mood disorders, with apathy, emotional flattening and indifference towards the environment, refered to as “pseudodepression”. A 14-year-old adolescent is transferred from a pediatric ward for frontal headaches, sleepiness, apathy, food refusal, irritability and marked weight loss (BMI = 14 kg/sqm). The patient has a history of Socialized Conduct Disorder, with extremely low compliance towards treatment. When admitted he is cooperating partially, has an influenced general state and refuses to drink liquids. He is sad, impulsive, with low frustration tolerance, negativist, oppositionist, with voluntary urine emissions and marked sleepiness. There are clinical signs of dehydration and an intermittent convergent strabismus in the left eye. Laboratory tests show an inflammatory syndrome, nitrate retention, dyselectrolytemia. Neurologically: exaggerated tendon reflexes, frust bipyramidal syndrome, slight ptosis of the left eye; electroencephalogram–slow activity (lesion?) in left deviations. A consult with the Infectious Disease unit renders a diagnosis of headache syndrome and frontal sinusitis. The MRI is suggestive for a left frontal infectious expansive process (abscess) and massive maxillary–ethmoidal–frontal sinusitis. Combined parenteral antibiotics and pathogenetic treatment are initiated and the patient undergoes neurosurgery with the evacuation of the tumor. A cystic formation of 6/5/1, 5 cm, containing an opalescent yellow liquid is found at the histopathological exam. Streptococcus spp. is identified by the bacteriological exam. The evolution is good under treatment, with a slight accentuation of the behavioural symptoms. This case illustrates the importance of correct differential diagnosis, the psychiatric diagnosis being one of exclusion.