Radiographic frames used for longitudinal studiesmay be in part unreadable for measuring crestal bone change. Sites may not be present on the film, or the measurement reliability may be compromised because of dissimilar geometry. Several techniques used to address this problem are expensive, time‐consuming, and require great skill. For the present study a commercially‐available alignment system was simply modified by addition of a reference pin in the bite block, facilitating the repositioning of the film holder for a second exposure. This study determined the ability of the modified instrument to: 1) improve the geometrical correspondence between serial radiographs; and 2) reduce the frequency of missed sites in the film. Two pairs of x‐rays were taken for each of 40 subjects, 1 pair with the standard alignment instrument of an assigned site and 1 pair with the modified instrument of the contralateral site. Measurements of alveolar bone height were performed using the “side by side” technique. The modified instrument yielded significantly smaller measurement differences and a significantly better geometrical correpondence than the conventional system (P<0.05). Also, the modified instrument yielded significantly greater (P<0.01) readable sites (86%) as compared to the conventional instrument (62%). The simply‐modified instrument facilitates the correct interpretation of serial radiographs.J Periodontol 1994;65:62–67.