The elevated serum alpha fetoprotein (AFP) concentration in ataxia-telangiectasia (A-T) patients has been known for decades, but the individual variation of AFP levels over time has not been studied. We have followed 12 patients (five girls and seven boys) for 1-12 years (mean 5.5 years) measuring in each patient AFP 2-8 (mean 4) times. Serum AFP levels were increased in all patients, mean 168.7 (range 40-373) kU/L, and without significant differences between the patients. There was a significant age related difference in the serum AFP level. A positive linear relationship (r=0.61, p=0.04) could be found between AFP level and age. Albumin levels were within normal range and did not change with age. Four patients had slightly increased aspartate aminotransferase (AST) levels. None of the patients had serological evidence of infectious hepatitis, and none had increased levels of carcinoembryonic antigen. Repeated standardized observations of gait function revealed no major difference in neurological deterioration between our patients. All had classical A-T disease and mainly truncating mutations; 21 out of 24 possible mutations were either frameshift or nonsense. Four were homozygous for the Norwegian ATM founder mutation. No correlation between serum AFP levels and the different ATM genotypes could be found. We conclude that serum AFP is not only elevated, but also is continuously increasing with age in patients with classical A-T disease.