D-2-(6'-Methoxy-2'-naphthyl)-propionic acid (naproxen) is a relatively well-known non-hormonal anti-inflammatory agent useful for long-term usage. The efficacy of naproxen in controlling the signs and symptoms of rheumatoid arthritis had been uniformly demonstrated in an extensive series of international double-blind trials lasting from a few weeks to four months. This paper deals with our experience with naproxen after 5 1/2 years of continuous usage. During this period of time ten different studies were conducted at the Rheumatology Departments of the General Hospital, Mexico City. The efficacy and tolerance of naproxen have been studied in the following indications: rheumatoid arthritis, osteoarthritis, acute and chronic gout and non-articular rheumatism. In these studies the usual clinical methods were used and some special objective methods were utilized in some others, for example, enzymology, scintigraphy, histopathology and arthroscopy. Some of these studies were comparative ones vs. some other well-known non-steroidal anti-inflammatory compounds such as aspirin, indometacin and ibuprofen. During the last 5 1/2 years 877 patients with different theumatic conditions were treated and observed in our Department and in the private practice. In 365 patients with rheumatoid arthritis excellent results were obtained in 34%, good in 36%, fair in 24% and negative in only 6%. In 358 patients with osteoarthritis excellent results were obtained in 28.2%, good in 46.6%, fair in 20% and negative in 5.3%. In 15 patients with acute gout using naproxen as exclusive therapy, excellent results were observed in nine patients very good in four, and unsatisfactory in only two cases. In 35 patients with chronic gout excellent results were observed in 22, good in 11 and negative in two. 93 patients out of the 877 studied since June 1969 used naproxen continuously as anti-rheumatic therapy for more than one year; 33 for more than two years; four patients for more than three years and five of the patients in a few days will complete 5 1/2 years. The side effects of the compound observed during this long period of time in these 877 patients on the g.i. tract, CNS, etc., although present in a limited number of cases (in less than 10% of total number of patients) were definitely less than those observed usually during anti-inflammatory therapy.