This paper develops a method of assessment of market access difficulties with an application to trade patterns between developing and developed countries. The method also offers a renewal of the assessment of the impact of regional trading arrangements. We use a micro-founded gravity-type model of trade patterns to estimate the impact of national borders on revealed access to Northern markets by Southern producers. Everything else equal, in the nineties, a rich country imports on average 276 times more from itself than from a developing country, only 31 times more when importing from another rich country. Results reveal that those difficulties faced by developing countries’ exporters in accessing developed countries’ consumers are furthermore higher than the reciprocal. Currently, the tariff equivalents of those border effects differ by around 30 percentage points. This asymmetry gets up to 50 points when considering trade between rich countries and lower middle income ones. Those considerable difficulties in Northern market access have however experienced a noticeable fall since the mid seventies. Another of our results concerns the impact of tariffs on market access. While tariffs still have in general an influence on trade patterns, our estimates suggest that they are an important component of market access difficulties faced by Southern exporters on Northern markets.