American researchers conducted a systematic review of the use of Echinacea spp. in treating the common cold. A total of 322 articles related to Echinacea spp. and colds, including nine placebo-controlled clinical trials, were identified using the Medline and Pubmed databases. Eleven features of experimental design that affect the accuracy of the measurement of features of interest, the probability of a chance relationship, bias and blinding were used to evaluate the nine placebo-controlled studies. The criteria were validated case definition, quantifiable hypothesis, sample-size calculation, randomised assignment, double blinding, proof of blinding, measurement of compliance, measurement of dropout rate, analysis by ITT, description of the methods of analysis and measurement of probability. Equal weight was given to each criterion since failure to meet any one of them could potentially invalidate the findings of a clinical trial. Of the nine studies, two met all 11 criteria. The results of both studies were judged to be negative by the people who performed the studies. Of the remaining seven studies, six were judged to have positive results, and one was judged to have negative results. The criterion most commonly not met was proof of blinding.