The expression of insulin-like growth factors I (IGF-I) and II (IGF-II) and insulin-like growth factor-I receptor (IGF-IR) was studied in 137 clear cell, 23 chromophobe, and 20 papillary renal cell carcinomas (RCCs) using a tissue microarray technique. IGF-I immunoreactivity was detected in 110 (82.1%) of 134 clear cell, 8 (36%) of 22 chromophobe, and 3 (15%) of 20 papillary RCCs (P < .001). IGF-IR immunoreactivity was detected in 39 (29.5%) of 132 clear cell, 9 (41%) of 22 chromophobe, and 19 (95%) of 20 papillary RCCs (P < .001). In contrast, all tumors lacked IGF-II expression. Expression of IGF-I and IGF-IR was not related to tumor stage, grade, or prognosis. The IGF system is expressed differentially among different tumor types. The expression of IGF-I together with its receptor, IGF-IR, provides evidence for the existence of an autocrine-paracrine loop of tumor cell stimulation in RCC and makes this type of cancer a candidate for therapeutic strategies aimed to interfere with the IGF pathway.