Diphenylcyclopropenone (DPCP) is widely considered the most effective topical immunotherapy for refractory or extensive alopecia areata (AA), but questions regarding how long to try DPCP therapy before terminating and what factors are prognostic of therapeutic success still remain unanswered. In this retrospective study of 50 AA patients, we evaluated DPCP efficacy and identified patient factors predictive of therapeutic success/failure. The median duration of DPCP treatment was 3 years, with 47% patients experiencing their first regrowth in the first 6 months of DPCP therapy, 20% between 6 months-1 year, and 8% between 1–2 years. In our study, treatment success, defined as ≥50% terminal hair regrowth, was reached in 71% of alopecia totalis patients and in 56% of alopecia universalis patients. Three factors were statistically significant predictors of poor treatment outcome—extent of hair loss before DPCP treatment, history of thyroid disease, and extent of body hair involvement. Relapse was observed in 44% of patients and significantly associated with history of thyroid disease. Common side effects were itching, rash, and local lymphadenopathy. The results of this study support our belief that DPCP therapy is a viable treatment option, can be successfully accomplished at home, and should not be terminated before 2 years.