The suppressor of cytokine signaling (SOCS) family of proteins has been implicated in the negative regulation of several cytokine pathways, particularly the receptor-associated tyrosine kinase/signal transducer and activator of transcription (Jak/STAT) pathways of transcriptional activation. SOCS-1 (also known as JAB and SSI-1) inhibits signaling by many cytokines. Because of the previously observed hypermethylation-associated inactivation of SOCS-1 in hepatocellular carcinoma and the critical role of interleukin-6 (IL-6) as a survival factor in multiple myeloma (MM), we examined CpG island methylation of the SOCS-1 gene in MM cell lines and primary MM samples. Aberrant SOCS-1 methylation was found in the IL-6-dependent MM cell lines U266 and XG1, which correlated with transcriptional silencing. Treatment of these cell lines with the demethylating agent 5-aza-2'-deoxycytidine (DAC) up-regulated SOCS-1 expression. Methylation-associated inactivation of SOCS-1 in hematopoietic cell lines correlated with greater sensitivity to the chemical JAK inhibitor AG490. Using methylation-specific polymerase chain reaction (MSP), we found that SOCS-1 is hypermethylated in 62.9% (23/35) of MM patient samples. In contrast, methylation analysis of malignant lymphomas of various histologies revealed SOCS-1 hypermethylation in only 3.2% (2/62), and there was no methylation of SOCS-1 in normal peripheral blood leukocytes or bone marrow cells. We conclude that SOCS-1 is frequently inactivated by hypermethylation in MM patients. Silencing of the SOCS-1 gene may impair negative regulation of the Jak/STAT pathway and therefore result in greater responsiveness to cytokines, thus supporting survival and expansion of MM cells.