The shoot apical meristem functions to generate the lateral organs of a plant throughout the vegetative and reproductive phases. Homeobox genes play key roles in controlling such developmental programs, but their modes of action have not been well defined. Here we describe isolation and biological functions of a novel tobacco homeobox gene, designated NTH15 (Nicotiana tabacum homeobox 15), from a tobacco shoot apex cDNA library. NTH15 encodes a polypeptide of 342 amino acids, its homeodomain is very similar to the class 1 KNOTTED-type homeodomains. NTH15 mRNA is mainly localized in corpus cells in the tobacco shoot apical meristem, but not in tunica layers nor in differentiated lateral organs. The NTH15 cDNA was fused to the cauliflower mosaic virus 35S promoter and used to generate transgenic tobacco plants. Almost all transgenic tobacco plants showed abnormal leaf and/or flower morphology, and were categorized into three groups depending on severity of the leaf phenotype. In transgenic leaves, drastic decrease of GA1 and increase of cytokinin were observed, while the levels of other phytohormones were only slightly changed. Taken together, our results suggest NTH15 is involved in tobacco morphogenesis and abnormal leaf morphology in transgenic plants results from altered hormone levels.