Forced vital capacity (FVC) and diffusing capacity for carbon monoxide (DLco) studies in two patients with proved interstitial fibrosis after low doses of bleomycin were contrasted with these measurements in 21 patients who received bleomycin without toxicity. The FVC decreased significantly (20% or more) in both fibrosis patients and in seven others. In one fibrosis patient and in three others the results could be explained by weakness. The DLco fell in both patients with fibrosis and in 12 others. Correction for decrease in hemoglobin level accounted for the change in one of the toxic patients and seven of the others, but hemoglobin correction made two insignificant changes significant and increased therapy values above control four times. Since these tests have many false-positive results and can be affected by weakness and anemia leading to false-negative results their use is ineffective and may be misleading.