In Southeast Asia, large forest fires occur during El Nino years, and smoke from these forest fires reduces the amount of photosynthetically active radiation (PAR). This study evaluated the reduction in net primary production (NPP), associated with the large reduction in PAR in this region, resulting from smoke. NPP was estimated from a slightly modified light-use-efficiency model using satellite-derived PAR. The results suggest that the reduction in NPP was large when heavy smoke occurred. On the islands of Kalimantan and Sumatra, the estimated reduction in NPP owing to smoke in 1997 was 0.12 (PgC/yr), which is approximately 3.5% of the total NPP in Southeast Asia (20N--10S, 90E--130E) and 6.8% of the total NPP in Kalimantan and Sumatra. This reduction in NPP influences the interannual variation in NPP on Kalimantan and Sumatra. Our results show the importance of including the effect of PAR reduction owing to smoke when assessing NPP in Southeast Asia.