Side effects of iohexol lumbar myelography have been analyzed with respect to the influence of the type of radiological abnormality, sex and age in a group of 200 patients. Headache, postural headache, nausea and back/leg pain were significantly more frequent in patients without definite radiological abnormalities. Postural headache, nausea, dizziness and mental symptoms were more frequent in women, while headache, postural headache, nausea, dizziness, minor mental symptoms (i.e. anxiety or depression) and pain became less frequent with age. This pattern is similar to that reported after lumbar puncture. Young women without definite clinical signs of nerve root lesions probably have the greatest risk of experiencing side effects after iohexol lumbar myelography.