Vascular lesions due to subclavian and internal jugular vein puncture may result in hematomas, which are usually clinically evident. While mostly benign, some of these hematomas can cause compression of the surrounding structures. When the hematoma is obvious, straightforward correlation can be made between the symptoms, for instance nerve compression, and the clinical signs. We present a case where we missed the diagnosis of phrenic nerve paralysis, which occurred after an unsuccessful, but apparently atraumatic attempt to puncture the internal jugular vein, prior to cardiac surgery. At the time the diagnosis was made (8 days post-op), the radiographic appearance of the neck was normal, and further investigation (i.e., CT-scan) had become pointless. A retrospective study of serial chest X-rays disclosed a space occupying lesion in the right lateral neck that displaced the nasogastric tube. This abnormality could only be seen on the first film and disappeared on the following. Since phrenic nerve paralysis is extremely rare in our institution, even after cardiac surgery, and as there was no clinical evidence of hematoma, our attention was not been drawn to the only definite sign that could have led to an early diagnosis.