Isoniazid is an efficacious treatment for latent tuberculosis. Concerns remain, however, regarding hepatotoxicity associated with this medication. In addition, adherence may be suboptimal because at least 6 months of treatment is required. We extracted information from our latent tuberculosis treatment database to determine adverse effects and treatment completion rates associated with the use of isoniazid at a county tuberculosis clinic. Outcomes were available for 3,788 patients started on isoniazid between 1999 and 2002. Six hundred seventy-two patients (18%) experienced one or more adverse effects, including 10 (0.3%) determined to have isoniazid-associated liver injury. No hospitalizations or deaths occurred in patients experiencing an adverse effect. A higher incidence of adverse effects was associated with increasing age. Sixty-four percent of patients completed at least 6 months of isoniazid. Higher completion rates were associated with younger age, Hispanic ethnicity, and non-U.S. country of birth. Lower completion rates were associated with being homelessness, using excess alcohol, and having experienced an adverse effect. In summary, we conclude that in our clinic population isoniazid is a safe therapy for latent tuberculosis, but its effectiveness is limited by modest completion rates.