Recently, p16 and p15 have been identified as commonly inactivated tumour suppressor genes in haematological malignancies. We previously reported that these genes were frequently hypermethylated in multiple myeloma (MM). To investigate how p16 and p15 inactivation are associated with hypermethylation, methylation status and transcription of these genes in six MM-derived cell lines were studied by Southern blot analysis and RT-PCR. Aberrant methylation of p16 was found in ARH-77, HS-Sultan, IM-9, RPMI-8226, U266-B1 and NCI-H929 MM cell lines. However, loss of p16 transcription was demonstrated only in HS-Sultan, RPMI-8226, U266-B1 and NCI-H929 with extensive methylation at the 5' upstream region of p16. Conversely, only HS-Sultan showed extensive methylation at the 5' upstream region of p15, which was associated with p15 transcriptional block. These results suggest that extensive methylation within a critical domain may be crucial in silencing p16 or p15 transcription. To demonstrate the reversibility of methylation and its relationship with transcription, HS-Sultan, RPMI-8226 and NCI-H929 were demethylated with 5-aza-2'-deoxycytidine. Restoration of gene transcription was observed and correlated with partial demethylation of the genes. The present data show that the p16 and p15 genes are silenced in MM by hypermethylation, which may play an important role in MM pathogenesis.