One-fifth of the gene mutations causing ornithine transcarbamylase deficiency cannot be detected. In such cases carrier risk estimation must refer to biochemical data'such as increased plasma glutamine concentration or increased orotidine excretion after allopurinol load —although these parameters do not yield a definite diagnosis. Here, I derive odds for carrier risk estimation from published data, i.e. from mean and standard deviation of glutamine concentrations in carriers and noncarriers, assuming normal distributions, and from allopurinol test results in individual carriers and noncarriers using logistic regression. I show how such biochemical information may be combined with genetic information, thus demonstrating the usefulness of biochemical data. The necessity to assess individual results in larger proband groups and to consider possible correlations between different parameters is indicated.