Primary intraocular lymphoma (PIOL) is a rare presentation of lymphoma that is particularly difficult to recognize. In our institution, 36 cases of PIOL were diagnosed between March 1997 and July 2002. The recognition of lymphoma cells by cytology with or without immunophenotyping on slides generated a strong suspicion of the diagnosis in 34 of 36 cases. The diagnosis was confirmed by measurement of interleukin-10 (IL-10) in the vitreous humour or aqueous humour; high levels were observed in 35 of 36 cases, all were of B-cell origin. As expected, the only case with T-cell lymphoma had a very low level of IL-10. Furthermore, IL-10 levels excluded this diagnosis in two cases that were incorrectly suspected of PIOL after cytological examination. Finally, detection of clonality by polymerase chain reaction techniques, performed in 29 cases, represented a helpful tool in diagnosing PIOL as this approach definitively confirmed the diagnosis of B- or T-cell lymphoma in 17 cases.