We have searched the human genome for genes that predispose to type 1 (insulin-dependent) diabetes mellitus using semi-automated fluorescence-based technology and linkage analysis. In addition to IDDM1 (in the major histocompatibility complex on chromosome 6p21) and IDDM2 (in the insulin gene region on chromosome llpl5), eighteen different chromosome regions showed some positive evidence of linkage to disease. Linkages to chromosomes llq (IDDM4) and 6q (IDDM5) were confirmed by replication, and chromosome 18 may encode a fifth disease locus. There are probably no genes with large effects aside from IDDM1. Therefore polygenic inheritance is indicated, with a major locus at the major histocompatibility complex.