The performance of the handheld Partector Pro has been evaluated against a reference mobility particle size spectrometer (MPSS) for 70 days at an urban background site. The number concentrations measured with the Partector Pro were in excellent and the number size distributions in reasonable agreement with the MPSS. The Partector Pro is thus a promising candidate for the widespread measurement of ultrafine particle concentrations as, for example, requested by WHO, at reasonable costs.