The diagnosis of allergic fungal sinusitis (AFS) is difficult to establish. The clinical presentation is not diagnostic. We define current criteria for diagnosis and the role of total and specific immunoglobulin E (IgE) and immunoglobulin G (IgG) levels in the disease process and review the clinical features of this disease. Fifty-one cases were identified in 44 patients who demonstrated characteristic allergic mucin. Thirty-one of these cases satisfied rigid criteria for the diagnosis of AFS. Twenty of these cases were classified as AFS-like syndrome. The prevalence of strictly defined AFS was 4.0%. Dematiaceous fungi were the predominant agents cultured. All our patients had polyps, 23 (54%) had asthma, 12 (27%) had aspirin sensitivity, 20 (65%) had eosinophilia, and 9 (69%) had increased total IgE levels.