Three hundred ninety-eight tuberculosis patients with tubercle bacilli resistant to isoniazid and/or streptomycin were matched by age, race, sex, and geographic area to an equal number of patients with tubercle bacilli susceptible to 9 drugs, including isoniazid and streptomycin, in an effort to determine whether the risk of infection and disease among contacts of patients with resistant bacilli is different from the risk among contacts to patients with susceptible bacilli. The risk of infection among contacts of previously untreated patients was not significantly different, regardless of whether the bacilli were drug-resistant or susceptible. However, the risk of infection increased if the index patient with resistant bacilli had been previously treated. We found no evidence of a lower risk of infection among contacts exposed to bacilli resistant to the highest concentration of isoniazid tested or among contacts exposed to bacilli resistant to both isoniazid and streptomycin. There was a strong association between infection risk among contacts and the age of the index case; younger patients were more infectious. Index cases tended to infect most (or all) or few (or none) of their contacts. The investigation of contacts of patients excreting drug-resistant bacilli should be given high priority.