International guidelines recommend the use of population-specific reference values to eliminate the well-recognized influence of ethnic variation on lung function. This study was designed to derive spirometric prediction equations for healthy Omani adults. Forced vital capacity (FVC), forced expiratory volume in one second (FEV(1)), peak expiratory flow rate (PEFR), and forced expiratory flow at 25% to 75% of FVC (FEF(25-75%)) were measured in 419 "healthy" nonsmoking Omani adults (256 men, 163 women), aged 18-65 years. Multiple linear regression analysis was performed for each spirometric parameter against age, height, and weight for men and women separately, and prediction equations for all the above parameters were derived and compared with values derived using equations published from other populations. All measured spirometric parameters increased with height and decreased with age, and they were all significantly higher in men. In contrast, FEV(1)/FVC% values decreased with height and increased with age and were higher in women. The predicted normal values of FVC and FEV(1) for our subjects using the derived equations were lower by 7-17% compared with respective Caucasian values, with smaller difference in the predicted values of PEFR, FEV(1)/FVC%, and FEF(25-75%). This report presents previously unavailable spirometric reference equations for the Omani adults. Our findings highlight the need to use reference values based on updated data derived from relevant populations.