The length of an open reading frame (ORF) is one important piece of evidence often used in locating new genes, particularly in organisms where splicing is rare. However, there have been no systematic studies quantifying the degree of correlation between length of ORF, on the one hand, and likelihood of gene function, on the other. In this paper, techniques are derived to estimate the conditional probability of gene function, given ORF length, based on evidence both from the databases and from simulation. Several complete chromosomes of Saccharomyces cerevisiae have now been sequenced, and considerable effort is being expended on locating and characterizing the genes in these sequences. Thus, we illustrate the techniques for this organism.