From January 1982 to December 1989, we experienced seven multilocular cystic renal cell carcinomas (MLCRCCs) in 36 asymptomatic renal cancers incidentally diagnosed, and none in 23 symptomatic renal cancers (p less than 0.05). No multiocular cystic nephromas (MLCNs) appeared in either group. Though MLCRCCs have been considered to be, in general, extremely rare, they do occur in asymptomatic renal cancers. A multiloculated renal mass with thick septum discovered in an adult by ultrasonography or CT scan should be suspected as being a MLCRCC rather than a MLCN.