The lowest observed adverse effect level for vitamin D, said to cause hypercalcaemia in normal adults, is officially 95 microg/day. Serum 25-hydroxyvitamin D (25[OH]D) concentrations associated with hypervitaminosis D remain undefined. Reported 25(OH)D concentrations resulting from prolonged excessive vitamin D3 intakes have exceeded 700 nmol/L. We report self-prescribed high dose of vitamin D3 over 5-6 years by two men. Subject 1 had been taking 100 microg/day for 3 years followed by 3 years of 200 microg/day. Serum 25(OH)D concentrations averaged 130 nmol/L while taking 100 microg/day of vitamin D3. While taking 200 microg/day of vitamin D3, mean serum 25(OH)D concentrations were 260 nmol/L with no hypercalcaemia or hypercalcuria over the 6 years of vitamin D3 intake. Subject 2 was a 39-year-old man diagnosed with multiple sclerosis. He initiated his own dose-escalation schedule. His vitamin D3 intake increased from 200 to 2200 microg/day over 4 years. The first evidence of a potential adverse effect was that urinary calcium:creatinine ratios showed an increasing trend, which preceded serum calcium concentrations above the reference range (2.2-2.6 mmol/L). His serum 25(OH)D concentration was 1126 nmol/L when total serum calcium reached 2.63 mmol/L. He stopped vitamin D3 supplementation at this point. Two months later, all biochemistry values were within reference ranges; serum 25(OH)D concentrations fell by about one-half, to 656 nmol/L. These results help to clarify the human response to higher intakes of vitamin D3. Close monitoring of biochemical responses confirmed that an increase in urinary calcium:creatinine ratio precedes hypercalcaemia as serum 25(OH)D concentrations rise.