Reiter's syndrome (nonspecific urethritis, conjunctivitis, and arthritis) may present with arthritis alone. There are, however, other discriminating clinical features that contribute to diagnosis. Additionally, the presence of HL-A W27 provides a new diagnostic aid. Thirteen patients were studied and support the validity of the concept of incomplete Reiter's syndrome. Peripheral arthritis was the presenting complaint in all, and none had urethritis or conjunctivitis. The majority were young white men. The arthritis was oligoarticular and asymmetric with lower extremity involvement predominantly. Heel pain was a prominent symptom in 10, with periostitis of other sites in 5, and "sausage digits" in 8. Mucocutaneous lesions occurred in 1 and significant weight loss in 6. Only 2 had sacro-illiitis. Chronicity has characterized the course in 12. HL-A W27 was present in 12 of 13. Additional data suggest that Reiter's syndrome may be one of the most common forms of inflammatory arthritis in young men.