The use of qualitative eco-evaluation methods for product concept eco-evaluation is explored in a case study where mechanical engineers performed an eco-evaluation of product concepts in a two-stage evaluation process. In the first stage of the case study, the evaluators were asked to evaluate the environmental friendliness of the product concepts and to rank the concepts accordingly. The obtained rankings were based upon subjective preferences of the evaluators and their subjective interpretation of the eco-value of the considered concepts. In the second stage of the case study, the evaluators eco-evaluated the same concepts, but this time they used ecodesign guidelines as eco-criteria. The evaluators have obtained different rankings of the concepts, so the results of the two sets of rankings are compared. Findings are analysed in the context of confounding variables of the case study design.