136 students of both sexes aged 14-18 years who were deprived of regular dental care participated in a 22-month longitudinal study. All students underwent scaling of the assigned teeth in 2 quadrants according to a randomised split-mouth design on contralateral quadrants. 2 groups of students were formed, one group received oral hygiene instruction (OHI). Follow-up examinations on gingival bleeding and calculus were carried out 6, 12, and 22 months after the scaling session. The prevalence of calculus on assigned teeth at baseline was high with a mean score of 1.10. At the end of the study, new calculus formation on scaled teeth reached a mean score of 0.58. Formation of calculus still continued on non-scaled teeth, but at a lower rate than on scaled teeth. Scaling resulted in an approximately 20% reduction of the gingival bleeding score which remained during the 22 months follow-up period. OHI had no significant effect on the calculus and bleeding scores. Since the effect of scaling alone on the gingival condition was small and the effect of a single OHI negligible, the practice of occasional scaling without repeated OHI, which is commonly employed in developing countries, should be considered as clinically irrelevant and of little use in improving the standard of periodontal health.