Inherent features of the PCR make this procedure suboptimal for quantitative applications. For typical clinical specimens, the absolute amount of product derived from PCR does not always bear a consistent relationship to the amount of target sequence present at the start of the reaction. Competitive PCR approaches to quantitation of nucleic acid sequences overcome the limitations of basic PCR methods for quantitation. We have developed a competitive PCR method known as quantitative competitive PCR for quantitation of HIV DNA and RNA sequences. Key features of the technique include quantitation, based on the relative amounts of products produced from the target sequence and a competitive template introduced into test specimens, and stringent internal control of all reactions, including the reverse transcription step in RNA PCR. The method is suitable for analysis of clinical specimens and may be particularly valuable for accurate quantitation of viral load in patients undergoing treatment with experimental therapies.