To determine the acute effect of smoking on DLCO, we studied 12 smokers (mean age, 36 yr; range, 19 to 52 yr, six men and six women) before and after they had smoked as many cigarettes as they could (mean, 6.0; SD, 1.9) over a period of 1 h. Blood COHb was estimated using a rebreathing-breathholding technique after a vital capacity inhalation of O2. Capillary blood volume (VC) was determined from DLCO performed at inspired O2 concentrations of 25 and 90%. DLCO (in ml/min/mm Hg) corrected for COHb back pressure decreased from 22.5 (SD, 6.6) before smoking to 21.0 (SD, 6.6) after smoking (p = 0.003). After correction for the "anemia effect" of COHb, DLCO still significantly decreased, from 22.8 (SD, 6.3) before smoking to 21.8 (SD, 6.4) after smoking p = 0.01). VC (corrected for the reduction in hemoglobin by COHb) was 52.0 (SD, 20.1) ml before smoking and 46.4 (SD, 22.7) ml after smoking; this difference did not achieve statistical significance (p = 0.056). There was no significant change in DLCO or VC in six control subjects tested before and after 1 h of sham smoking of an unlit cigarette. In 12 control subjects studied before and after inhalation of 0.1% CO to result in mean COHb levels of 10.6% (SD, 1.4%), there was a slight but significant decrease in VC (mean change, 21%) and in DLCO (mean change, 4%) after correction for COHb back pressure and reduction in available hemoglobin, suggesting that CO inhalation may have a direct effect on DLCO by reducing VC.(ABSTRACT TRUNCATED AT 250 WORDS)