SOLANUM is the largest genus in the Solanaceae whose arrangement into infra-generic taxa remained controversial for more than 150 years. A total of 41 characters of leaf morphology, distribution of spines, hair types and crystalline cell inclusions in floral and vegetative parts were recorded for a sample of 51 species of Solanum and Lycianthes rantonnetii. The data matrix was subjected to numerical analysis under 56 combinations of seven dissimilarity measures and eight clustering methods to select the dendrogram which best reflects the relationships among the species in terms of the recorded data. Solanum is divisible into two main groups which correspond roughly to the two traditional subgenera Leptostemonum and Archestemonum. The group resembling Leptostemonum takes in all the species with 3-lobed, shallowly or deeply dissected, partially pinnate, pinnate and bi-pinnate leaves with maximum blade incision index from 27% to 100%, whereas the group comparable with Archestemonum comprises all the species with only simple leaves and maximum incision index ranging between 0% and 18%. Of the 25 species in the former group, 20 are woody herbs or shrubs furnished with sharp spines on at least the stem, leaf petioles, leaf veins or the persistent calyx or a combination of two or more of these parts; most members of the latter group are spineless. Separation of Lycianthes rantonnetii from Solanum is not supported by the present study. Epidermal hairs and crystalline cellular inclusions have little distinctive value for groups and subgroups of Solanum.