Sensex fell by over 31 points in opening trade today as investors locked in gains registered during the previous four sessions. The 30-share barometer, which rallied by nearly 840 points the previous four sessions, slid to 18,271 points in the first few minutes of trade as realty, oil and gas and capital goods stocks succumbed to profit-booking. In a similar fashion, the wide-based National Stock Exchange Nifty index declined by 7.90 points to 5,474 points. Hong Kong's Hang Seng index was up by 0.17 per cent, while Japan's Nikkei gained 0.69 per cent in early trade today. The US Dow Jones Industrial Average ended 0.50 per cent higher in yesterday's trade.