PURPOSE Although the treatment of symptomatic convergence insufficiency (CI) with base-in (BI) prism in adults has a strong theoretical foundation, there have been very few studies addressing its efficacy. The purpose of this study was to investigate whether the application of BI prism, using a novel progressive addition lens design which incorporates BI prism in the near portion only, could help alleviate the symptoms in presbyopes with CI. METHODS A total of 29 symptomatic CI subjects aged 45 to 68 years were examined. All subjects took the CI Symptom Survey V-15 (CISS) and scored >or=16 points. Each subject was assigned two pairs of progressive addition glasses made by the same manufacturer in a randomized sequence, one with BI prism and one with the same lens prescription but no prism (placebo). Subjects wore each pair of glasses for 3 weeks and completed the CISS at the end of the 3rd week. Symptom level measured with CISS was the major outcome measure. RESULTS The mean (standard deviation) CISS score was 30.21 (9.30) at baseline and decreased to 13.38 (9.44) with the BI-prism glasses, vs. 23.62 (10.76) with the placebo glasses. There were significant differences between the baseline survey score and the score with the BI-prism glasses (p < 0.0001) and between the score with placebo glasses and the one with BI-prism glasses (p = 0.001). CONCLUSIONS The progressive addition glasses with BI-prism were found to be effective in alleviating symptoms of presbyopes with symptomatic CI.