Unique Chlamydia trachomatis strains characterized by multiple nonfusing inclusions were recently described. These strains lack evidence of the protein IncA in the inclusion membrane and have mutations in the incA gene. This study evaluated the epidemiology and clinical manifestations of patients infected with nonfusing mutant strains (case patients) and compared them with patients infected with wild-type fusing strains (control subjects). Both male and female case patients had fewer signs of infection than did control subjects (P = .016 and P = .019, respectively). Female case patients also had fewer symptoms of infection (P = .02). Median inclusion-forming unit (ifu) counts were lower in male and female case patients (P = .045 and P = .135, respectively). Thus, nonfusing strains of C. trachomatis more often produce subclinical infections than do normal fusing strains and have lower median ifu counts. From a prevention perspective, the data underscore the importance of screening programs to detect and treat inapparent C. trachomatis infection. © 2001 by the Infectious Diseases Society of America.