The importance of body size in predicting many aspects of an animal's biology has become well established in recent years. However, little is known about how body size evolves at the cellular level. Some published data suggest that it is cell number and not cell size that accompanies changes in organ and body size across taxa. We examined organ and cell allometry in the wing, eye and basitarsus of adult Hawaiian Drosophila, ranging in body length from 0.2 mm to 0.8 mm. Linear measurements of all three structures exhibit a positive allometry with body length. Exponents of the allometric equation were 0.96, 0.55 and 1.50 for wing, eye and basitarsus, respectively. Surface markers were used to quantify cell size of each organ. The allometric exponents for cell size as a function of organ size were 0.53, 0.68 and 0.33 for wing, eye and basitarsus, respectively. In contrast to reports in the literature on other systems, our results for Hawaiian Drosophila indicate that cell size may contribute between one third and two thirds to evolutionary changes in organ and body size.