This article uses panel data estimation techniques to examine the relation between the number of federal court civil filings by inmates and jail and state prison populations (and, hence, the relation between jail and prison inmate filing rates) both before and after the effective date, in 1996, of the Prison Litigation Reform Act (PLRA). The research issue matters for several reasons. First, the amount of litigation by inmates is a crucial component of the regulatory regime governing jails and prisons and thus what factors drive filings, and by how much, deserves close attention and assessment. In addition, the PLRA was a major congressional attempt to control and ration litigation; understanding its effects in finer gauge seems itself worthwhile. Finally, we hope to show, methodologically, how research about litigation rates can be carried out sensitively, even if the litigation results from case filings by two separate populations. We make three major findings. (1) As expected, inmate filings vary positively with prison population. However, the relationship with jail population is less secure. (2) As the prison proportion of inmates in a particular state increases, so too does the number of filings. (3) The PLRA’s passage has significantly lessened but not eliminated this prison proportion effect.