The study population consists of individuals who were previously enrolled in raltegravir expanded access program(EAP) and completed at least 8 weeks of treatment with raltegravir, whose chart from EAP program is available for review, and whose resistance testing prior to initiation of raltegravir is available.

Inclusion Criteria:

• Patients previously enrolled in the MK 0518 EAP are eligible

• Patients not enrolled in the MK 0518 EAP (or other Raltegravir protocols) but who meet the specific EAP protocol entry criteria are eligible:

  • Age >= 18 years
  • Limited or no treatment options due to resistance or intolerance to multiple antiretroviral regimens, documented resistance to at least one drug in each of the 3 classes of oral ARTs (NRTI, NNRTI, PI) by genotype or phenotype testing, intolerance defined as having had a clinically significant adverse event which in the opinion of the clinician provides a contraindication to the use of any drug in that class iii. Patient did not achieve virologic suppression on ART regimen prior to receipt of raltegravir iv. Patient was clinically stable at time of initiation of raltegravir, eg. clinical status and all chronic medications (except ARTs) unchanged for >= 2 weeks prior to raltegravir receipt.
• Patient received raltegravir for at least 8 weeks
• Baseline and week 8 or later HIV viral load done and available for review
• Resistance test (either genotypic or phenotypic test) available prior to receipt of raltegravir

Exclusion Criteria:

• Patient did not receive approved raltegravir dose of 400 mg BID for at least 8 weeks.
• Patient chart not available for review.