To assess the influence of contrast medium on cortical function, we studied 20 patients undergoing lumbar myelography with iopamidol and 10 patients undergoing diagnostic lumbar puncture (controls). The examinations performed before and 6 and 24 h after myelography (or lumbar puncture) included a neuropsychological battery and an electrophysiological evaluation. In the patients cranial CT was performed thrice to assess passage of contrast medium from the cerebrospinal fluid into the brain. Neither patients nor controls had significantly different scores on neuropsychological testing after the diagnostic examinations. A transient slowing of basal EEG activity could be detected in 2 patients and 3 controls 6 h after the lumbar puncture. In 3 patients CT showed a transient increase in density of the brain. None of the parameters studied was significantly affected by myelography with iopamidol. CT findings support the hypothesis of early clearance of iopamidol from brain tissue, explaining its low neurotoxicity.