Amiodarone-induced interstitial pneumonitis is a serious, frequently fatal untoward effect of a commonly used antiarrhythmic agent. Recent reports suggest that bronchoalveolar lavage (BAL) fluid cellular analysis might be used to diagnose amiodarone-induced pneumonitis. The purpose of this study was to determine if the diagnosis of amiodarone-induced pneumonitis could be made by patient history, pulmonary function evaluation, and examination of BAL fluid. We studied five groups of patients. Three of the five groups received amiodarone: patients receiving amiodarone without evident lung toxic reaction, patients with amiodarone-induced pneumonitis, and amiodarone-treated patients diagnosed as having other pathologic processes involving the lung. The two other groups examined were healthy volunteers and patients with interstitial lung disease from causes other than amiodarone. Pulmonary function tests included vital capacity (FVC), first second forced exhaled volume (FEV1), total lung capacity (TLC), and diffusing capacity for carbon monoxide (DCO). BAL fluid analysis included total and differential cell counts. We found that amiodarone-induced interstitial pneumonitis was not associated with an alteration in pulmonary function or BAL cellular composition which could permit its distinction from amiodarone-treated patients diagnosed as having an unrelated pulmonary process or patients with interstitial lung disease from other causes. The most frequent abnormality encountered in patients with amiodarone toxicity was a reduction in the percentage of macrophages in the differential cell count. The sensitivity, specificity, and predictive value of this finding was 82 percent, 69 percent, and 69 percent, respectively. The sensitivity, specificity, and predictive value of a > or = 15 percent reduction in DCO was 44 percent, 50 percent, and 36 percent, respectively. We conclude that amiodarone-induced interstitial pneumonitis remains a diagnosis of exclusion, and the role of BAL fluid analysis is to narrow the differential diagnosis through microbiologic culture and cytologic examination.