1 alpha(OH) vitamin D3 at the dose of 6 micrograms per week was given for 4 weeks to 16 stable patients on chronic hemodialysis with a low dialysate aluminum while taking a constant dose of Al(OH)3. A significant increase of their plasma aluminum was observed from 1.2 +/- .25 mumol/l before 1 alpha(OH)D3 to 1.7 +/- .35 during the second fortnight of 1 alpha(OH)D3 administration and this increase surprisingly was maintained at 1.71 +/- .3 up to 6 weeks after 1 alpha(OH)D3 discontinuation. Increases in plasma calcium and decreases in plasma PTH were observed during 1 alpha(OH)D3 administration and these changes were correlated to the changes in plasma aluminum. It is concluded that the increase in plasma aluminum observed with 1 alpha(OH)D3 and after its discontinuation is either due to body aluminum burden redistribution or to increased aluminum intestinal absorption whatever the mechanism is, this effect should lead to close monitoring of plasma aluminum in uremic patients taking 1 alpha OH vitamin D3.