During a 4-year period, 1978-1981, 34 patients with bilateral rhegmatogenous retinal detachment were operated on at the University Eye Hospital in Helsinki. The incidence of bilaterality in the entire detachment population was 10%, in the aphakic group it was 16%. In 24 cases (71%) the interval from the first to the second eye detachment was less than 5 years, 6 patients (18%) had bilateral detachment simultaneously. The mean age of the patients when the first eye was affected (46 years, range 6-73) was significantly younger than the mean age of those with unilateral detachment (58 years, range 6-83) (P less than 0.001). Previous eye diseases were significantly (P less than 0.01) more common in patients with bilateral than in those with unilateral detachment, but the incidences of myopia, aphakia and lattice degeneration of the retina did not differ significantly between these groups. At least one of these predisposing factors was found in 85% and two or more of them in 53% of bilateral detachments. The retina was re-attached in 80% of the 44 eyes operated on during the study period. Of the 24 eyes operated on earlier, 71% were blind (visual acuity CF 1 m or worse). The latest visual acuities in both eyes or in the better eye for all patients were: greater than or equal to 0.5 in 38%, 0.4-0.2 in 35%, 0.1-CF2 m in 12%, and less than or equal to CF1 m in 15%.