In vitro and in vivo studies have shown that various strains of “Viridans streptococci” (nongroupable α-hemolytic streptococci) inhabiting the oropharynx suppress the growth of gram-positive and gram-negative microorganisms. We conducted an inventory of the oropharyngeal flora from ambulatory asthma and chronic obstructive pulmonary disease (COPD) patients and a control group to examine the interaction between viridans streptococci and potential pathogens in vivo. In addition, the difference in colonization patterns of these bacteria was studied. Oral washings from 195 patients, 48 asthma (24.6%), 147 COPD (75.4%), and 157 control subjects were examined microbiologically on two occasions with a 2-wk interval, resulting in a total of 384 and 295 oral washings, respectively. All patients were in a stable phase of disease throughout the study. The distribution of low (≤ 104/ml) or high (≥ 105/ml) concentrations of viridans streptococci did not differ substantially between asthma or COPD patients and control subjects. Potentially pathogenic microorganisms found in a low (≤ 104/ml) or high (≥ 105/ml) concentration were equally distributed between the two groups. Staphylococcus aureus and β-hemolytic streptococci were found significantly less often in the asthma and COPD group (p < 0.005 and p < 0.0005, respectively), but the prevalence of Enterobacteriaceae species was significantly higher (p < 0.0005). No correlation was found between the concentration of viridans streptococci and the prevalence of gram-negative microorganisms. These findings suggest that viridans streptococci are probably not responsible for growth control of gram-negative microorganisms in vivo. Another as yet unknown factor may be responsible for suppressing the growth of both gram-negative microorganisms and viridans streptococci.