Tumour-related tissue reactions resulting in the formation of epithelioid-cell granulomas have been known for almost 70 years. Such sarcoid reactions may occur in lymph-nodes draining an area housing a malignant tumour, in the tumour itself, and even in non-regional tissues. Overall, sarcoid reactions occur in 4.4% of carcinomas, in 13.8% of patients with Hodgkin's disease, and in 7.3% of cases of non-Hodgkin lymphomas. Similar histologic changes in sarcoma appear to be extremely rare. Most probably, sarcoid reactions are caused by antigenic factors derived from the tumour cells, eliciting an immunological hypersensitivity reaction leading to the formation of epithelioid-cell granulomas. Sarcoid reactions may be a marker of an immunologically mediated antitumour response of macrophages activated by T-lymphocytes, and in Hodgkin's disease there is evidence that patients with sarcoid reactions have a better prognosis. On occasion sarcoid reactions may be so extensive that they complicate the diagnosis of an underlying malignant disease. Problems may also arise of distinguishing between tumour-related sarcoid reactions and true systemic sarcoidosis.