Despite vast knowledge on liver regeneration, little is known about the effect of active liver atrophy and regeneration on tumor growth. Ligation of a branch of the portal vein to the one or two anterior lobes was performed in inbred Wistar rats. This induces acute atrophy of the anterior and regeneration of the other lobes. During the same operation a tumor cell suspension (NGW1--adenocarcinoma) was inoculated in liver lobes undergoing atrophy and regeneration. Tumor volume and weight were measured and the histologic appearance was assessed. During the early and active phases the tumor growth was significantly accelerated in regenerating lobes and partially inhibited in rapidly atrophied segments. After the regeneration and atrophy was completed the normal pattern of growth was re-established in both parts of the liver. The results suggest that tumor growth is affected in proportion to regenerative response. They further suggest that portal branch ligation is of limited value in surgical palliation of liver tumors. The risk for further induction of growth of clinically undetected tumor foci in the remaining liver tissue appears to be small, although a significant, but short-lasting, stimulatory response was found.