To determine healthcare access and costs for triply diagnosed adults, we examined baseline data from the HIV/AIDS Treatment Adherence, Health Outcomes and Cost Study, a multi-site cohort study of HIV+ adults with co-occurring mental and substance abuse disorders conducted between 2000 and 2004. Baseline interviews were conducted with 1138 triply diagnosed adults in eight predominantly urban sites nationwide. A modified version of Structured Interview for DSM-IV Axis I Disorders (SCID) was used to assign Diagnostic and Statistical Manual of Mental Disorders, 4th edition (DSM-IV) diagnoses for the preceding year. Utilization of a broad range of inpatient and outpatient services and medications over the preceding three months was patient-reported in face-to-face interviews. We then applied nationally representative unit costs to impute average monthly expenditures. We measured (poor) access to care during the three-month period by whether the patient had: (a) no outpatient medical visits; (b) at least one emergency room visit without an associated hospitalization; and (c) at least one hospitalization. At baseline, mean expenditures were $3880 per patient per month. This is nearly twice as high as expenditures for HIV/AIDS patients in general. Inpatient care (36%), medications (33%), and outpatient services (31%) each accounted for roughly one-third of expenditures. Expenditures varied by a factor of 2:1 among subgroups of patients, with those on Medicare or Medicaid, not in stable residences, or with poor physical health or high viral loads exhibiting the highest costs. Access to care was worse for women and those with low incomes, unstable residences, same-sex exposure, poor physical or mental health, and high viral loads. We conclude that HIV triply diagnosed adults account for roughly one-fifth of medical spending on HIV patients and that there are large variations in utilization/costs across patient subgroups. Realized access is good for many triply diagnosed patients, but remains suboptimal overall. Deficiencies in HIV care are unevenly distributed, tending to concentrate on already disadvantaged populations.