Eight randomly selected patients undergoing the Outerbridge-Kashiwagi (ulnohumeral arthroplasty) debridement procedure for primary osteoarthritis of the elbow had fenestration of the olecranon fossa by use of a bone trephine. The cores of bone removed were compared histologically with age- and sex-matched controls derived from necropsy samples with no history of osteoarthritis. All components of the olecranon fossa membrane (anterior cortical bone, medullary cavity, posterior cortical bone, and anterior and posterior fibrous tissue) were noted to be of increased thickness in those patients with osteoarthritis of the elbow when compared with the control group. With the exception of the anterior surface fibrous tissue, these differences were of statistical significance by use of the Wilcoxon signed rank test (anterior cortical bone, P =.01; medullary cavity, P =.01; posterior cortical bone, P =.02; and posterior surface fibrous tissue, P =.01). Bone volume was also measured in the two groups and was statistically greater in the patients with osteoarthritis compared with the control group (P =.01).