BACKGROUND The efficacy of antiperspirants is a current topic among the developers of cosmetic products. According to the Food and Drug Administration (FDA) for the US market, efficacy testing performed in the axilla of human volunteers is mandatory. Another method is yet available, which enables comparison of more than one antiperspirant formula in a single study by performing the test on the backs of volunteers. However, how reproducible are these methods, comparing between the back and axilla? Do they differ as a result of seasonal variation? Is a correlation between the results of the two methods possible? METHODS To answer these questions, the antiperspirant efficacy of aluminium chlorohydrate (ACH) aqueous solutions was investigated in the axilla and on the backs of volunteers, in four separate clinical studies covering cold and warm seasons. Four days of product application were followed by thermal sweat induction on the fifth day, using a sauna. The amount of sweat recovered by weighing cotton pads before and after sweat induction was used to calculate sweat reduction. Testing in the axilla and on the back was performed on the same volunteers simultaneously to achieve the best comparable data. For this reason, the FDA guideline was slightly modified to thermal stimulation in a sauna instead of in a hot room. RESULTS Increasing concentrations of ACH in aqueous solutions on the backs of volunteers showed a saturation for 8% ACH with a sweat reduction of approximately 50%. The antiperspirant efficacy of solutions containing 4%, 8% or 12% ACH was repeatedly found at the same levels, when tested on the backs during summer, autumn and winter time. Axilla tests, with an 8% ACH aqueous solution, showed strongly varying results for summer and winter time, represented by sweat reduction values of -2% to 25%. As an assumption, these high variations might result from reduced gel formation in cold seasons due to low humidity in the axillae during the application phase. On the back, this effect was avoided by applying occlusive foils after product application. To gain further insight, a study, during which summer conditions were artificially simulated by thermal stimulation during the application phase, again showed decreased antiperspirant efficacy in the axilla for winter conditions with sweat reduction values of 2%, compared with 25% under simulated summer conditions. CONCLUSION These strongly varying values of sweat reduction in the axilla under summer and winter conditions make comparisons between antiperspirant products difficult and a statement about correlation between the two test sites back and axilla impossible. A standardization of the application phase, comparable to the simulated summer conditions described here, could be a solution to reduce the high variation of results in the axilla. Consequently, testing on the back is not only a more cost-effective method to investigate the antiperspirant efficacy of more than one formulation, but a reproducible method more independent of climatic influences during test implementation than the axilla test method. It could, therefore, be regarded as the method of choice for discriminating antiperspirant efficacy between several products during development of new antiperspirant formulations.