Fifty-three commercial New Guinea Impatiens cultivars (Impatiens hawkeri Bull.) from six different breeding series were tested for level of heat tolerance. Five floral (flower number, flower length, flower width, floral dry weight, and flower bud number) and five vegetative characteristics (leaf dry weight, stem dry weight, total dry weight, number of nodes, and number of branches) were evaluated with emphasis placed on continued flowering under long term heat stress. Significant differences among cultivars were found in each data category (P ≤ .0001). Flower number varied from 0 to 6, flower length varied from 10 to 51 mm, flower width varied from 10 to 47 mm, floral dry weight varied from 0 to 0.5 g, and flower bud number varied from 0 to 14. Four heat tolerant (Celebration Cherry Red, Celebration Rose, Lasting Impressions Shadow, and Paradise Moorea) and three nonheattolerant (Lasting Impressions Twilight, Danziger Blues, and Pure Beauty Prepona) cultivars were identified using a Weighted Base Selection Index. These cultivars were used as parents in a full diallel crossing block with reciprocals and selfs. One hundred seedlings from each of 49 crosses were evaluated for heat tolerance. General and specific combining abilities of the parents were evaluated as was heritability. It was found that the four heat tolerant cultivars had higher general combining abilities. Heat tolerance has low heritability and is controlled by many genes. Superior genotypes were identified (selection intensity of 0.05) and retained for further evaluation and breeding efforts.