Our objective was to determine the yield and cost of standardized laboratory testing of HIV-infected patients entering medical care after testing positive for HIV. An HIV staging and referral clinic in a municipal public hospital was our site for a cross-sectional study, and 308 patients were evaluated in the clinic between February 1, 1990 and October 1, 1991. Patients underwent standardized laboratory testing, including hematologic studies, serum chemistries, infectious disease serologies, and chest radiographs. The percentage of abnormal results for each test was determined. Abnormal results were stratified as mild or severe. They were also examined with regard to whether injection drug users or other patient subgroups had higher percentages of abnormalities. Charges and Medicare reimbursements for the tests were also determined. There were substantial numbers of abnormalities in all types of laboratory testing. Only 3% of patients had normal CD4 lymphocyte counts; 85% had counts of3, and 35% were3. Forty-four percent of patients had at least one abnormal hematologic study; 8% were severe. Nearly 75% had abnormal liver function tests; 20% of these were severe abnormalities. Fifteen percent of patients were PPD-positive, and >50% were anergic. Fourteen percent had a positive nonspecific test for syphilis, and 7% had a positive confirmatory test. Fourteen percent of patients had an abnormal chest radiograph. Injection drug users were more likely to have severely abnormal liver function tests (25.7% vs. 14.7%;p< 0.001), be hepatitis B core antibody positive (88.6% vs. 62.0%;p< 0.001) and have a positive syphilis serology (19.8% vs. 6.8%;p< 0.001) compared with non drug-users. Total average charges for the laboratory panel were ∼422 per person, and Medicare reimbursements were 198. In a municipal public hospital clinic, extensive initial laboratory testing of HIV-infected patients yields a substantial number of abnormalities, many of which are severe and require clinical intervention. Routine laboratory testing of patients like these entering primary care for HIV infection is justified, given the high frequency of abnormalities in all groups of patients.