Some examples of lobular carcinoma in situ (LCIS) may be composed in part of signet ring cells. Such proliferations have been considered examples of pleomorphic LCIS based on pathological features of the more conventional component. However, the occurrence of LCIS composed entirely of signet ring cells is extraordinarily rare. This report describes an example of an in situ proliferation that was composed almost entirely (>95%) of signet ring cells, which was unassociated with an invasive carcinoma and which showed comedo-type necrosis. There was only focal lobulocentric distention by lesional cells, as is typical of classic LCIS. However, discrete, ductal-type cross-sectional profiles showed a purely intraepithelial proliferation of remarkably discohesive signet ring cells. The signet ring cells had intermediate-grade nuclear atypia, no significant mitotic activity and were positive for mucicarmine and PAS stains (the latter with and without diastase predigestion). The cells displayed marked immunoreactivity for high-molecular-weight keratin (stained by 34β E12 antibody), MUC1, gross cystic disease fluid protein-15, cytokeratin 7 and were negative for cytokeratin 20, E-cadherin, progesterone receptor and HER2/neu. It is concluded that this is an example of a purely signet ring variant of pleomorphic LCIS.