The base values of electrooculograms (EOGs) of 39 patients with new, unilateral rhegmatogenous retinal detachment and clear media were studied. None of the patients had a history of ocular surgery. Ophthalmoscopic examinations revealed choroidal detachment in 11 eyes. The mean (+/- SD) amplitude, expressed as a percentage of the amplitude recorded for unaffected fellow eyes, was 116 +/- 31% in the 28 eyes without choroidal detachment, and 39 +/- 10% in the 11 eyes with choroidal detachment. This difference was statistically significant (P < 0.0001). There was no significant correlation in either group between baseline value and extent of retinal detachment or intraocular pressure. These results indicate that choroidal detachment associated with rhegmatogenous retinal detachment can be revealed electrophysiologically by the baseline value of an EOG.