In the Sudano-sahelian zone, which includes Northern Cameroon, the inter-annual variability of the rainy season is high and irrigation is scarce. As a conse- quence, bad rainy seasons have a detrimental impact on crop yield. In this paper, we assess the risk mitigation capacity of weather index-based insurance for cotton farmers. We compare the ability of various indices, mainly based on daily rainfall, to increase the expected utility of a representative risk-averse farmer. We first give a tractable definition of basis risk and use it to show that weather index-based insurance is associated with a large basis risk. It has thus limited potential for income smoothing, whatever the index or the utility function. Second, in accordance with the existing agronomical literature we find that the length of the cotton growing cycle, in days, is the best performing index considered. Third, we show that using observed cotton sowing dates to define the length of the grow- ing cycle significantly decreases the basis risk, compared to using simulated sowing dates. Finally we found that the gain of the weather-index based insurance is lower than that of hedging against cotton price fluctuations which is provided by the national cotton company. This casts doubts on the strategy of international institutions, which support weather-index insurances in cash crop sectors while pushing to liberalisation without recommending any price stabilization schemes.