Recently, the occurrence of cyclin D1-positive B cells with mantle cell lymphoma phenotype in the inner mantle zones of morphologically inconspicuous lymph nodes has been described and termed mantle cell lymphoma 'in situ'. Prevalence and clinical significance of this lesion and related minimal mantle cell lymphoma infiltrates in reactive lymphoid tissues of healthy individuals, and of mantle cell lymphoma patients are unknown. All 1292 reactive lymph nodes from unselected consecutive surgical specimens of 131 patients without a history of lymphoma obtained over a 3-month period were stained for cyclin D1. In addition, all morphologically reactive lymph nodes and benign-appearing extranodal lymphoid infiltrates of patients diagnosed with mantle cell lymphoma in the years 2000-2011 were studied. Samples predating the lymphoma diagnosis for at least 2 months were available from 37/423 (9%) patients. A mantle cell lymphoma 'in situ' was not identified in any of the two groups. However, in four patients with subsequent mantle cell lymphoma diagnosis, an early manifestation of mantle cell lymphoma was detected retrospectively, antedating the lymphoma diagnosis for 2-86 months. In six mantle cell lymphoma patients, only small groups of cyclin D1-positive cells in morphologically reactive extranodal infiltrates were detected >2 months before the diagnosis of mantle cell lymphoma (range 3-59 months). Mantle cell lymphoma 'in situ' is an extremely rare phenomenon in morphologically reactive lymph nodes, in line with the low prevalence of t(11;14)-positive cells described in the peripheral blood of a healthy population. In mantle cell lymphoma patients, however, immunohistochemically detectable infiltrates of mantle cell lymphoma cells antedating the lymphoma diagnosis were found in a significant proportion of cases (10/37=27%). These consisted either of early mantle cell lymphoma with mantle zone growth pattern, or small extranodal accumulations of cyclin D1+ cells, whereas typical mantle cell lymphoma 'in situ' was not detected.