The frequency and incidence of certain forms of toxicity in patients receiving long term chrysotherapy for rheumatoid arthritis have been reliably calculated for defined, sequential time periods. Ninety-four patients who received 100 courses of gold therapy for a total treatment period of 134.4 patient-years were followed at one gold therapy clinic. For the period of 0--3 months, the incidences of rash, mouth ulcer, and proteinuria were 9.8, 4.0, and 1.8 episodes per 10,000 patients/month, respectively. However, the incidence of these forms of toxicity decreased progressively in subsequent time periods of continued chrysotherapy. No similar decrease was noted in the incidence of thrombocytopenia, but it would appear that the methodology used in monitoring significantly affects the incidence of clinically important thrombocytopenia. No predictive correlates could be determined for patients who had gone into sustained remission. However, the data strongly suggest that patients who improve within 6 months may continue chrysotherapy for at least up to 3 years with an increasing margin of safety for mucocutaneous and renal toxicity.