At entry, patients are randomized to Arm A (sequencing) or Arm B (phenotyping) and have a resistance test drawn while still receiving the current regimen even though regimen failure is suspected. The test results are available between Weeks 1 and 4, inclusive. There are weekly visits for the first 4 weeks after entry to monitor viral load and maintenance of the current failing (prestudy) regimen. If virologic failure is confirmed, a new regimen is chosen and prescribed at the first visit after resistance test results are available. [AS PER AMENDMENT 12/6/00: If the resistance assay fails to yield results, another regimen is chosen and prescribed based on the patient's medical and medication history.] If virologic failure is not confirmed, the current drug regimen is not changed. Otherwise, on-site study visits occur every 4 weeks until Week 24 and then every 8 weeks thereafter through Week 48. [AS PER AMENDMENT 12/6/00: on-site study visits occur every 4 weeks until Week 24 and then every 8 weeks thereafter]. Medical resource use is assessed at baseline and then every 8 weeks through Week 48. Quality of life is assessed at baseline and then every 16 weeks through Week 48.