Dormant solid tumors were produced in vivo by prevention of neovascularization. When small fragments of anaplastic Brown-Pearce carcinoma were implanted directly on the iris in susceptible rabbits, they always vascularized. A characteristic growth pattern, consisting of prevascular, vascular, and late phases, was observed, which terminated with destruction of the eye within 2 wk. The beginning of exponential volume increase was shown to coincide with vascularization of the implant, as demonstrated by perfusion with intravenous fluorescein and by histologic sections. In contrast, implants placed in the anterior chamber, at a distance from the iris, did not become vascularized. After initial growth into spheroids, they remained arrested at a small size comparable to prevascular iris implants, for periods as long as 6 wk. Although dormant in terms of expansion, these avascular tumors contained a population of viable and mitotically active tumor cells. When reimplanted on the iris, vascularization was followed by rapid, invasive growth. These observations suggest that neovascularization is a necessary condition for malignant growth of a solid tumor. When a small mass of tumor cells is prevented from eliciting new vessel ingrowth from surrounding host tissues, population dormancy results. These data suggest that the specific blockade of tumor-induced angiogenesis may be an effective means of controlling neoplastic growth.