Alternethanoxins A (1) and B (2) are fungal phytotoxins that are produced by Alternaria sonchi and have been recently characterized as new polycyclic ethanones. Triacetyl (3) and dimethyl (4) derivatives of compound 1 were evaluated together with alternethanoxins for their in vitro growth inhibitory activities in five human and one mouse cancer cell lines in comparison to the reference compound temozolomide (TMZ). Compounds 1-4 and TMZ displayed similar growth inhibitory activities, and these anticancer activities were equivalent in cancer cell lines that display certain levels of resistance to pro-apoptotic stimuli and those that are sensitive to pro-apoptotic stimuli. Of the six cancer cell lines under study, the human esophageal cancer cell line OE21 was the most sensitive to the four polycyclic ethanones. Computer-assisted phase-contrast microscopy (quantitative videomicroscopy) revealed that compounds 1, 2 and 4 displayed cytostatic rather than cytotoxic growth inhibitory effects, while compound 3 appeared to have cytotoxic effects. Thus, this study creates a stimulus for further structure-activity investigations with respect to the anticancer activities of compounds belonging to the alternethanoxin group. The observed toxicity does not seem to be affected by the stereochemistry of C-6 of the B ring, the presence of a hydroxy group at C-1 or the presence of a furan ring joining rings A and C in alternethanoxin B. The anticancer activity (cytostatic versus cytotoxic) of this type of compound could be affected by the chemical moieties present at the hydroxy groups at C-4 and C-6, as was observed for the cytostatic and cytotoxic activities of derivatives 4 and 3, respectively.