To evaluate the association between in-vitro resistance of herpes simplex virus type 2 to acyclovir and breakthrough recurrences of herpes despite chronic suppressive therapy, we determined the in-vitro sensitivity of herpes simplex virus isolated before, during, and after therapy. One hundred eighty-three virus isolates from 107 patients were tested. Before therapy, the median amount of drug required to inhibit 50% of the virus in tissue culture (ID50) was 0.91 microgram/mL. The median ID50 after therapy was 0.99. Six isolates from patients with culture-positive breakthrough recurrences were evaluated. The median ID50 was 0.90 microgram/mL (range, 0.39 to 1.55). The development of breakthrough recurrences could not be correlated with infection with strains of herpes simplex virus type 2 that were resistant to acyclovir in vitro. Acyclovir-resistant strains are not commonly recovered from patients during acyclovir therapy, nor does there seem to be a high frequency of resistance after 4 months of chronic suppressive therapy.