The red blood cell distribution width (RDW) was studied in 26 unselected patients with untreated pernicious anemia. RDW changes were also sequentially followed after therapy in 12 patients. The mean (+/- 1 SD) RDW values were significantly higher in pernicious anemia patients than in controls (21.7 +/- 9.1% vs. 13.2 +/- 1.1%, P less than 0.0001). Nevertheless, 31% of the patients had normal RDWs. There were no consistent findings among those who had normal RDW. Most of them were in the early stages of deficiency, but some had advanced deficiency. Over half of those with normal RDW also had normal mean corpuscular volume (MCV). Overall, 9 of the 26 patients (35%) had normal MCV. Of eight patients whose RDW fell with therapy, some showed a steady fall while others had a transient rise followed by a progressive drop. Despite current advocacy that a high RDW is a sensitive and consistent finding in vitamin B12 deficiency, our findings show that a large proportion of untreated pernicious anemia patients have normal RDWs and that in contrast to iron deficiency, elevation of RDW is not necessarily the earliest indicator of vitamin B12 deficiency.