Background. Evaluation of renal hemodynamics requires estimation of effective renal plasma flow, which is commonly measured by the renal clearance of p-aminohippuric acid (PAH). There are many existing methods for PAH assay that are either complicated, expensive or time consuming. Methods. We describe a rapid, precise and accurate microplate based assay of PAH using p-dimethylaminocinnamaldehyde that gives a red color on reacting with PAH and compare it to a reference HPLC method. Renal PAH clearances were measured in 10 volunteers and clearances calculated using the new and HPLC methods. Results. There was excellent agreement between the HPLC and the microplate method of PAH assay. The average ratio of microplate to HPLC method was nearly 1.0 and the limits of agreement (2SD) for plasma, urine and clearances were 17.2%, 19.3% and 25.5%, respectively. Intraday coefficient of variation (CV) for urine and plasma were <7%, interday CV was <6% for urine and plasma samples. Conclusions. The microplate method is a reliable alternative to a reference HPLC method and can be performed for a fraction of the cost, time and reagents.