We studied the infection rate in patients with rheumatoid arthritis (RA) treated with low-dose methotrexate (MTX) in a 6-year open prospective study and in a 12-month randomized double blind trial comparing MTX with azathioprine (AZA) that was followed by a 3-year open prospective study. The literature on infections during low dose MTX in RA was reviewed. We also did a search for therapy-related opportunistic infections in RA and in MTX-treated psoriasis and psoriatic arthropathy patients. In our studies the infection rate during MTX treatment was higher in severe RA than in moderate RA. In severe RA there were often 2 infections simultaneously. The majority of the infections occurred in the first 1.5 years of treatment. There was no difference in the infection rate of MTX and AZA in the comparative trial. In the literature the infection rate was highest in short-term double-blind studies. Opportunistic infections are increasingly reported in RA treated with MTX and rarely with AZA, cyclosporine A, and cyclophosphamide or in MTX treated psoriasis and psoriatic arthropathy. In RA it appears that the initial period of treatment with MTX is the most vulnerable phase for infections, with the exception of opportunistic infections, which are not limited to a certain treatment period. Probably there are more MTX-associated infections in severe RA than in moderate RA.