The difficulties presented by out-of-print books along with the library profession's concern about the problem are discussed, while the literature concerning Books in Print Plus and out-of-print book patterns is reviewed. In this investigation, the Books in Print Plus and Books Out-of-Print Plus CD-ROM databases were used to analyze the subject patterns whereby books become out-of-print. On two different occasions identical searches for a range of years in 39 different subject areas were conducted in the two databases in order to calculate in-print/out-of-print proportions, resulting in four sets of data; Analysis of the results revealed, contrary to expectations, remarkably little difference in the percentage of books in-print and out-of-print among the three major branches of knowledge: the sciences, the social sciences, and the humanities. A number of limitations to this approach are acknowledged, while issues for future research are suggested.