Background: Infection is a major cause of non-unions. Infection is not always evident clinically, nor on bacteriological analysis. If it is untreated, non-union treatment may fail. Aim: To establish whether histological analysis is of value as an adjunct to microbiology in establishing the diagnosis of infection. Methods: Sixty patients who had both bacteriological and histological analysis of their non-unions were studied. Infection was indicated by an acute inflammatory response. In 45 of the 60 fractures, microbiological and clinical diagnoses were in agreement; in this subset the histology results were compared to the established diagnoses. Results: The histological diagnosis for the 45 fractures in the group with a definite diagnosis gave four false negatives but no false positives. This represents a sensitivity of 87.1%, (95% CI, 70-96%) and a 100% specificity (95% CI, 77-100%). The overall accuracy was 91.1%. (95% CI, 79-98%) The predictive value of a positive test was 100% and of a negative test was 77.8%. In 25% of the series a definite diagnosis could not be made with purely clinical and microbiological information; with the additional histological information it was possible to determine the infection status of the non-union. Conclusion: Histology is of particular use when the microbiology is negative or equivocal.