The authors describe very rare complication (pseudopleural effusion) after peripheral venous cannulation (Introcan-W 24G3/4 through v. axilaris) in extremely premature newborn with birth weight 600 grams, which lead to worsening of patient's respiratory insufficiency. Ultrasonographic examination confirmed extrapleural process without necessity of surgical intervention. Cannula extraction and infusion interruption were followed by spontaneous regression of the complication, thereafter artificial ventilation was discontinued and baby was discharged.