It is known that the O2 COV in COLD is high; O2 administration to these patients lowers airway resistance, a major determinant of the COV. Thus, O2 should lower the COV. We measured the COV in ten stable COLD patients and five normal control subjects breathing room air and 30 percent O2. Results indicate that the COV of our patients was elevated above that of control subjects, was related to disease severity, and was decreased with 30 percent O2. The COV of control subjects also was lowered by O2. At rest, O2 lowered VE, VEQ O2 and HR. During submaximal exercise O2 lowered VE, reduced VEQ O2 and extended total exercise time. An inverse correlation was noted between COV and maximal O2 uptake. Thus, in stable COLD, the COV is elevated in proportion to the degree of airway obstruction, inversely related to exercise capacity and lowered by O2 administration.