A six-centre study was conducted to evaluate the effectiveness and safety of a single weekly treatment with 50 or 150 mg levamisole in patients with rheumatoid arthritis (RA). The results were compared with those of a previous placebo-controlled study, in which 150 mg levamisole was given on 3 or 7 consecutive days each week. The efficacy/side effect ratio is better with 150 mg once weekly than with 150 mg on 3 or 7 consecutive days each week. A reduction of the dose to 50 mg reduces the incidence of idiosyncratic reactions but also reduces efficacy. A single dose of 150 mg levamisole once weekly is therefore recommended as a standard dose in patients with RA. This regimen allows for hematological controls at critical times and, hence, permits recognition in practice of agranulocytosis at the incipient stage.