Software patents have grown rapidly in number and now comprise 15% of all patents. They are acquired primarily by large manufacturing firms in industries known for strategic patenting; only 5% belong to software publishers. The very large increase in software patent propensity over time is not adequately explained by changes in R&D investments, employment of computer programmers, or productivity growth. The residual increase in software patent propensity is consistent with a sizeable increase in the cost effectiveness of software patents during the 1990s, perhaps arising from changes in the application of patent law to computer software.