In order to study the process of hepatocellular carcinoma (HCC) development, and to search for a clue to histologic diagnosis of well-differentiated HCC (wd-HCC), interstitial invasion in small HCC was evaluated. The study material consisted of 35 cases of HCC that were smaller than 3 cm that comprised 17 cases of wd-HCC, 18 cases of moderately or poorly differentiated classical HCC (cl-HCC), and 20 cases of large regenerative nodules (LRN). Interstitial invasion was microscopically classified into three patterns: (i) crossing type, in which HCC was invading across fibrous septa of tumour nodules; (ii) longitudinal type, in which tumour cells were growing longitudinally within fibrous septa; and (iii) irregular type, in which the portal area was irregularly invaded by HCC. The crossing type was found in two cases (12%) of wd-HCC and 10 cases (56%) of cl-HCC while the longitudinal type was observed in 16 cases (94%) of wd-HCC and eight cases (44%) of cl-HCC. The irregular type was frequently seen in wd-HCC (15 cases, 88%), and cl-HCC (12 cases, 67%). No interstitial invasion was observed in LRN. Interstitial invasion could be recognized even in the low magnification view of histological specimens, with a detection rate of 59% (10 cases) in wd-HCC and 72% (13 cases) in cl-HCC. These results suggest that evaluation of interstitial invasion is useful in diagnosing wd-HCC independent of cellular atypia. In addition, such invasive growth is revealed to play an important role in destroying original hepatic architecture during its developmental process from the early to advanced stages.