Most mycobacterial lymphadenitis in children in developed countries is caused by non-tuberculous (the so-called 'atypical') mycobacteria. In view of the widely different treatment regimes and the requirement for contact tracing in Mycobacterium tuberculosis infections but not in non-tuberculous mycobacterial infections, it is very important to attempt to define histologically which is the aetiological agent. We have reviewed the histological appearances of mycobacterial cervical lymphadenitis in children and have found that, if any one of several 'atypical' features were seen, the appearances were much more likely to be due to a non-tuberculous mycobacterium. These features include ill-defined (non-palisading) granulomas, irregular or serpiginous granulomas, a predominantly non-specific granulomatous response, predominantly sarcoid-like granulomas or lack of significant caseation. In addition, the non-tuberculous mycobacterial infections showed a different distribution of neutrophil polymorphs, which tended to be seen in the centre of areas of necrosis rather than in Mycobacterium tuberculosis infections where a polymorph infiltrate, if present, was more diffusely scattered. Although no one definitive feature is diagnostic of non-tuberculous mycobacterial infection, some features are helpful in differentiating the two groups of organisms histologically.