To ascertain the value of screening for tuberculosis in the New York City (NYC) alcoholic and drug abusing welfare population, 2,641 clients were interviewed, and 970 (36.7%) of them met preestablished criteria for alcohol or drug abuse. The prevalence of active tuberculosis was 0.91%, which is 28 times the age-matched NYC rate. Screening only those persons with a positive PPD and a cough substantially increased the yield of active tuberculosis to 7.2%, or 225 times the NYC rate. The prevalence of a positive tuberculin skin test was 32.4%, or 1.5 times greater than the age-matched NYC rate. Treatment or prophylaxis for tuberculosis was required in 128 or 13.2% of the screened population. Seventy thousand NYC welfare clients are routinely evaluated for medical illness each year. This study predicts that in 1 yr this subpopulation could yield 239 clients with active tuberculosis and 3,181 requiring INH prophylaxis. Screening for tuberculosis in the alcoholic and drug abusing welfare clients is therefore urgently recommended.