OBJECTIVE: To evaluate the effects of zidovudine therapy in patients with mildly symptomatic HIV infection using Q-TWiST (quality-adjusted: Time Without Symptoms and Toxicity). DESIGN: Analysis of a previously reported multicenter, randomized, placebo-controlled clinical trial. SETTING: Thirty-two AIDS Clinical Trial units. PATIENTS: A total of 351 patients with mildly symptomatic HIV infection were assigned to placebo, and 360 patients were assigned to zidovudine, 1200 mg/d. MEASUREMENTS: A modified Q-TWiST method for comparing treatments based on time spent without severe symptomatic adverse events and without disease progression. Zidovudine and placebo were compared in a threshold utility analysis considering reduction in quality of life associated with adverse events and disease progression. Adverse events defined by laboratory findings were distinguished from findings representing symptomatic events. RESULTS: The incidence of severe symptomatic adverse events was 22.8% for the zidovudine group and 15.1% for the placebo group (P = 0.01), but, as previously reported, zidovudine improved progression-free survival relative to placebo (at 18 months, 91% compared with 81%; P = 0.001). In an 18-month period, patients receiving zidovudine went an average of 14.5 months without disease progression or a severe symptomatic adverse event compared with 14.7 months for placebo. The zidovudine group gained 0.9 months without disease progression but lost 1.1 months due to adverse events. Within the 18-month observation period, treatment provided more Q-TWiST than placebo if the quality of life after HIV disease progression was assumed to be 10% to 20% worse than the quality of life after a severe symptomatic adverse event. CONCLUSIONS: The Q-TWiST analysis projects that quality-of-life reductions due to severe symptomatic adverse events might be balanced by the quality-of-life benefits of delayed HIV disease progression for patients who received zidovudine for mildly symptomatic HIV infection. At currently recommended doses (500 to 600 mg/d, half the dose used in this study) zidovudine therapy is likely to yield a more favorable result.