The sera of 116 children with a variety of rheumatic diseases were assayed for the presence of antibodies to bovine native type I collagen by an enzyme-linked immunosorbent assay. For comparison, the sera of 69 children without rheumatic diseases, 50 healthy adult blood donors and 23 adults with rheumatoid arthritis (RA) were similarly assayed. Twenty-four % of the 90 children with juvenile rheumatoid arthritis (JRA) had antibodies to native type I collagen. Eleven of 26 (42%) with polyarticular onset, 5 of 53 (9%) with pauciarticular onset and 6 of 11 (55%) with systemic onset JRA had such antibodies. Seven of 10 children with spondyloarthropathies and 8 of 16 children with a variety of connective tissue diseases had antibodies to native type I collagen. Among children with nonrheumatic diseases, 12 of 69 (17%) were anticollagen antibody positive. Anticollagen antibodies were found in 4% of adult blood donors and 70% of adults with RA. Thus, antibodies to native type I collagen occur in a wide variety of childhood rheumatic diseases and their presence is not confined exclusively to rheumatic disorders.