The main objective of this paper is to evaluate the impact of the following four groups of factors on success perception by managers of Polish small-and medium-sized international new ventures (SMEs INVs): 1) competitive strategy expressed by international orientation of top management and applied marketingmix tools, 2) competitive strategy analyzed within M.E. Porter's framework, 3) sector-specific determinants (industrial branches), and 4) main export markets. Two measures of success perception are considered: subjective evaluation of firm's financial situation and assessment of firm's success as compared to competitors. The methods of data analyses are correlations and multiple linear regression (SPSS software). The analysis is conducted separately for smallsized companies and for medium-sized firms. The hypotheses concerning the roles of international orientation, marketing-mix tools, industrial branches, and main export markets in explaining success have been fully or partially confirmed. The factors that are most positively related to the perception of success across both groups of firms and two measures of success perceptions are high-quality products, good product development strategy and capability to create unique products, i.e. differentiation strategy. The obtained results imply that the most fundamental factor of success is a good product policy within the framework of a differentiation strategy. The second objective of this paper is to provide a comparison between small and medium-sized enterprises, analyzing whether there is a reason to consider SMEs as a uniform category, or whether the factors explaining success are different depending on firms' size.