Changes in DNA methylation patterns are an important characteristic of human cancer. Tumors have reduced levels of genomic DNA methylation and contain hypermethylated CpG islands, but the full extent and sequence context of DNA hypomethylation and hypermethylation is unknown. Here, we used methylated CpG island recovery assay-assisted high-resolution genomic tiling and CpG island arrays to analyze methylation patterns in lung squamous cell carcinomas and matched normal lung tissue. Normal tissues from different individuals showed overall very similar DNA methylation patterns. Each tumor contained several hundred hypermethylated CpG islands. We identified and confirmed 11 CpG islands that were methylated in 80-100% of the SCC tumors, and many hold promise as effective biomarkers for early detection of lung cancer. In addition, we find that extensive DNA hypomethylation in tumors occurs specifically at repetitive sequences, including short and long interspersed nuclear elements and LTR elements, segmental duplications, and subtelomeric regions, but single-copy sequences rarely become demethylated. The results are consistent with a specific defect in methylation of repetitive DNA sequences in human cancer.