Background In this working paper, a preliminary estimate of the immediate cost of chikungunya and dengue to the Indian state of Gujarat has been estimated by combining nine earlier studies on major cost factors such as costs of illness and control, and thus building a more comprehensive picture of the immediate cost of these Aedes mosquito-borne diseases to Gujarat. Methods Costs of illness and vector control comprise the immediate cost of chikungunya and dengue. In this working paper, cost of illness has been calculated using the RUHA matrix approach. Using the shares of reported (R) and unreported (U) hospitalised (H) and ambulatory (A) cases of chikungunya and dengue, a RUHA matrix has been constructed for the state of Gujarat. Cost of illness has been estimated by combining this matrix with ambulatory and hospitalisation costs per case and the number of reported cases. For this study, chikungunya and dengue were assumed to be identical from the point of view of disease control and management. Vector control cost includes state and municipal expenditure to prevent/control these diseases, a conservative fraction of the household insecticides market, and private sector cost. Comparisons with Asian countries have been used to estimate a parameter if direct data is unavailable. Monte-Carlo sensitivity analysis was carried out to find out how uncertainties in each cost parameter affected the total cost of chikungunya and dengue. Findings Using Monte-Carlo sensitivity analysis, the immediate cost of chikungunya and dengue to Gujarat has been estimated to be 3.7 (range 1.6-9.0) billion rupees per annum. This is a preliminary estimate; research is in progress to refine key parameters from the Monte-Carlo analysis such as ambulatory cost per case and reporting rate. The emotional and long-term burden of illness and deaths due to these diseases including impact on tourism, education, economic growth, per capita income, FDI, etc. are beyond the scope of this study. Extrapolating from Gujarat to the whole of India (after adjusting for the relative number of cases in each state and differences in state GSDP per capita), the immediate cost of chikungunya and dengue to the whole of India is approximately INR 61 billion (range INR 26-148 billion). Interpretation The annual cost of INR 3.7 billion (range INR 1.6-9.0 billion) translates to approximately INR 66 per capita (range INR 29-159), or US$ 1.6 (range US$ 0.7-3.8) per capita using an exchange rate 42 INR/US$. Comparable cost of dengue is US$ 5.3 in Malaysia and US$ 6.2 in Panama, while Brazil spends US$ 4.3 per capita on dengue prevention alone. The differences in these costs can be partially be explained by roughly five times higher GDP per capita in Malaysia, Panama and Brazil than in Gujarat. However, higher incidence of chikungunya increases the relative cost in Gujarat. As policy makers weigh investments in new technologies and expanded use of existing interventions to control neglected tropical diseases, the economic cost of illness is a major input into decision making. It is hoped that this preliminary estimate will trigger more refined studies on cost of illness as well as cost-effectiveness of vaccines and other interventions to combat these neglected tropical diseases