Calcitonin gene methylation at CCGG sites were determined in 39 chronic myeloid leukemia patients by isoschizomeric restriction endonuclease analysis. A total of 27 patients were analyzed while still in the chronic phase: 20 patients had a normal gene, and seven had a hypermethylated gene. There were 12 patients initially studied in accelerated or blastic phases. All but one patient showed gene hypermethylation, suggesting a good correlation between gene methylation and disease stage. All five patients who, while still in the chronic phase, had a major 3.1-kb hypermethylated calcitonin gene fragment, accelerated within 2 to 27 months. In consecutively analyzed patients, the initially normal calcitonin gene changed to a hypermethylated state as the disease escalated. The hypermethylation predicted disease acceleration with a median lead time of 6 months before any morphologic or clinical signs of disease progression were seen. The disease progressed in 8 of 27 patients initially studied in the chronic phase: in only two patients this occurred without predictive methylation changes. The results suggest that the assessment of calcitonin gene methylation status may be a promising tool for monitoring chronic myeloid leukemia disease escalation.