Organisms of the Mycobacterium avium-intracellulare complex (MAC) have been demonstrated to be susceptible to moxifloxacin. However, clinical data on how to utilize moxifloxacin to treat disseminated MAC are scanty. In addition, there have been no moxifloxacin pharmacokinetic-pharmacodynamic (PK/PD) studies performed for MAC infection. We utilized an in vitro PK/PD model of intracellular MAC to study moxifloxacin PK/PD for disseminated disease. Moxifloxacin doses, based on a serum half-life of 12 h, were administered, and the 0- to 24-h area under the concentration-time curve (AUC(0-24)) to MIC ratios associated with 1.0 log(10) CFU/ml per week kill and 90% of maximal kill (EC(90)) were identified. The AUC(0-24)/MIC ratio associated with 1.0 log(10) CFU/ml kill was 17.12, and that with EC(90) was 391.56 (r(2) = 0.97). Next, the moxifloxacin MIC distribution in 102 clinical isolates of MAC was identified. The median MIC was 1 to 2 mg/liter. Monte Carlo simulations of 10,000 patients with disseminated MAC were performed to determine the probability that daily moxifloxacin doses of 400 and 800 mg/day would achieve or exceed 1.0 log(10) CFU/ml per week kill or EC(90). Doses of 400 and 800 mg/day achieved the AUC(0-24)/MIC ratio of 17.12 in 64% and 92% of patients, respectively. The critical concentration of moxifloxacin against MAC was identified as 0.25 mg/liter in Middlebrook media. The proposed susceptibility breakpoint means that a larger proportion of clinical isolates is resistant to moxifloxacin prior to therapy. For patients infected with susceptible isolates, however, 800 mg a day should be examined for safety and efficacy for disseminated M. avium disease.