During 1984 and 1985 we performed frequent measurements of serum aluminum (Al) in patients with moderate chronic renal failure, and healthy controls, all living in the Oslo region. The results demonstrated seasonal variations with high levels in the autumn. During the peak periods serum Al increased by a factor greater than four. Outside the peaks patients using Al-containing phosphate binders had higher serum Al levels than non-users, a difference not seen during the peaks. Serum Al levels were unrelated to parathyroid hormone (PTH) concentrations and to calcitriol intake. Urinary excretion and the glomerular filtration rate was stable during the period with high serum Al in the autumn 1984. Increased gastrointestinal absorption of Al, possibly caused by a waterborne factor with chelating properties, may explain the seasonal variations.