Thirty patients of rheumatoid arthritis comprising 16 classical and 14 definite cases based on the ARA criteria were evaluated in a prospective and controlled study for iron status with special reference to serum ferritin levels. Serum ferritin levels were estimated by RIA technique and marrow iron status was ascertained by semi-quantitative estimation after Pearl's staining of marrow aspirate (G 0-6). Marrow iron stores were found absent to decreased in 17 patients (56.7%), normal in 2 (6.7%) and increased in 11 patients (36.6%). The serum ferritin levels in the iron depleted rheumatoid arthritis patients were significantly lower in comparison to patients with normal to increased marrow iron stores (23.91 +/- 11.45 ug/L vs 69.94 +/- 24.7 ug/L, p less than 0.001). There was a strong positive correlation between serum ferritin levels and marrow iron stores (r = +0.08, p less than 0.001). A serum ferritin value of less than or equal to 32 ug/L was a good predictor of decreased iron stores in the bone marrow, with a sensitivity of 88.2% and specificity of 84.5%. The test had a predictive value of 83.33%. There was no correlation between marrow iron stores and conventional indicators or iron status i.e. serum iron, TIBC, transferrin saturation and MCHC. It is concluded that serum ferritin correlates well with marrow iron stores and can be used as a simple non-invasive test for predicting iron-deficiency in patients of rheumatoid arthritis.