The existence of an in situ phase of malignant mesothelioma has long been postulated but until recently has been impossible to prove. Here we describe ten patients with mesothelioma in situ, defined by a single layer of surface mesothelial cells showing loss of BAP1 nuclear immunostaining, no evidence of tumor by imaging and/or by direct examination of the pleura/peritoneum, and no invasive mesothelioma developing for at least 1 year. Nine cases were pleural and one peritoneal. Most patients were biopsied for repeated effusions of unknown etiology; in two patients mesothelioma in situ was found incidentally in lung cancer resections. In addition to surface mesothelium with BAP1 loss, one case had a surface papillary proliferation with BAP1 loss, and two cases had a small (few millimeter) nodule with BAP1 loss. CDKN2A was deleted by FISH in one of eight cases. Methylthioadenosine phosphorylase showed partial loss in the surface mesothelium by immunohistochemistry in three cases. Invasive malignant mesothelioma developed in seven patients with time between biopsy and invasive disease from 12 to 92 (median 60) months. Invasive mesothelioma has not developed in the other three patients at 12, 57, and 120 months, but the latter patient, who has pleural plaques, still has repeated pleural effusions, probably representing a so-called "benign asbestos effusion." We conclude that mesothelioma in situ, as diagnosed using the criteria outlined above, is associated with a high risk of developing invasive mesothelioma, but typically over a relatively protracted time, so that curable interventions maybe possible.