1. Fractional fat clearance rate ( K 2 ) in 21 patients with colorectal cancer was studied using the intravenous fat tolerance test. 2. K 2 showed a negative correlation with weight change( r = −0.736, P < 0.001) with the tumour in situ. 3. Fourteen patients were retested 12 weeks after curative resection of their tumours. Eight of the fourteen patients had raised K 2 values pre-operatively; in all these the K 2 values were reduced post-operatively (pre-operative median 3.94%/min, range 2.63–7.20%/min, postoperative median 2.31%/min, range 1.57–5.36%/min, P < 0.01 by Wilcoxon's signed rank test). A high preoperative K 2 value was associated with a large reduction in K 2 post-operatively ( r = − 0.844, P < 0.001). 4. Seven patients with relatively high K 2 values received a pre-operative course of intravenous nutrition after which K 2 values were significantly reduced (pre-feeding median 7.20%/min, range 4.72–12.12%/min, post-feeding median 4.44%/min, range 2.17–6.83%/min, P < 0.05).