Loop-mediated isothermal amplification (LAMP) is a nucleic acid amplification method that allows the synthesis of large amounts of DNA in a short period of time with high specificity. As the LAMP reaction progresses, the reaction by-product pyrophosphate ions bind to magnesium ions and form a white precipitate of magnesium pyrophosphate. We designed an apparatus capable of measuring the turbidity of multiple samples simultaneously while maintaining constant temperature to conduct real-time measurements of the changes in the turbidity of LAMP reactions. The time (Tt) required for the turbidity of the LAMP reaction solution to exceed a given value was dependent on the quantity of the initial template DNA. That is, a graph with the plot of Tt versus the log of the amount of initial template DNA was linear from 2 x 10(3) copies (0.01 pg/tube) to 2 x 10(9) copies (100 ng/tube) of template DNA. These results indicate that real-time turbidity measurements of the LAMP reaction permit the quantitative analysis of minute amounts of nucleic acids present in a sample, with a high precision over a wide range, using a simple apparatus reported in this study.