To gain information concerning the natural history and prevalence of pulmonary gas exchange abnormalities resulting from intravenous drug abuse, 45 intravenous drug users were studied. Twenty subjects used a mixture of the synthetic opiate pentazocine and the antihistamine tripelennamine, which, under the street name T's and B's, has become very popular in some urban areas as an available substitute for heroin. Compared with the 19 heroin addicts studied, the pentazocine and tripelennamine users had a significantly shorter mean duration of intravenous drug abuse (2.7 +/- 0.4 years versus 7.6 +/- 0.9 years, p less than 0.01), a greater frequency of respiratory symptoms (75 percent versus 36 percent, p less than 0.05), a significant reduction in the mean diffusing capacity of the lung for carbon monoxide (58.4 +/- 3.3 percent predicted versus 75.5 +/- 5.6 percent predicted, p less than 0.01), and abnormal responses to submaximal steady-state exercise testing. The intravenous use of pentazocine and tripelennamine and probably most other drug preparations intended for nonparenteral use represents a particularly noxious form of drug abuse that may lead to early respiratory complications in a large proportion of users.