We examined 32 patients with cystic fibrosis, paying special attention to optic nerve performance and pupillary function. Decreased visual acuity occurred in nine of 64 eyes. Three of 17 patients (18%) who used chloramphenicol had bilaterally delayed P100 waves of the visual-evoked response of greater than 3 standard deviations. This was not found in patients who did not use chloramphenicol. Contrast sensitivity in patients with cystic fibrosis was decreased at every spatial frequency when compared to healthy controls. This decrease was noted in patients who did and did not use chloramphenicol, suggesting that chloramphenicol is not the only cause of decreased contrast sensitivity in cystic fibrosis. With pharmacologic pupil testing we determined that patients with cystic fibrosis display a preganglionic oculosympathetic paresis that corresponded to the disease severity, as measured by the Shwachman score.