In a case series, there were 4 total cases of PML in rheumatoid arthritis treated with rituxan out of 129,000 exposed, suggesting risk of 1/25,000; 1 case with no biologic and minimal immunosuppressive therapy.
“Rituximab-associated progressive multifocal leukoencephalopathy in rheumatoid arthritis JAMA Neurology, “09/14/2011 Clifford DB et al.
To my knowledge there have been no reported cases of PML in rituxan treated patients with multiple sclerosis; However, there are about 114 total cases in association with rituximab as of 2013
source: “New insights on human polyomavirus JC and pathogenesis of progressive multifocal leukoencephalopathy.”
Clin Dev Immunol. 2013;2013:839719. doi: 10.1155/2013/839719. Epub 2013 Apr 17.; Bellizzi A1, Anzivino E, Rodio DM, Palamara AT, Nencioni L, Pietropaolo V.
However, a lot of these patients received other immunosuppressive drugs as well.