The clinical significance of corrected sinus node recovery time (CSNRT) and the natural and unnatural history of sinus node dysfunctions are not completely known. To gain some insight into this problem, 101 patients (pts) (54M, 47F, mean age +/- SD = 62.02 yrs +/- 14.42) with clinical and ECG signs of definite or suspected sick sinus syndrome (SSS) underwent an electrophysiologic study and then were prospectively followed for a mean period of 44.36 months +/- 18.96 (range: 2-78 months). The pts were divided into two groups: 1) Group A: 68 pts with prolonged CSNRT (greater than 500 msec); 2) Group B: 33 pts with normal CSNRT. Thirty-three pts of Group A (48.5%) and 2 pts of Group B (6.1%) received VVI pacemaker implantation (PM) immediately after the electrophysiologic study. The following results were obtained: 1) Pts of Group A showed a higher prevalence of organic heart disease and of ECG signs of definite SSS than pts of Group B. (p less than 0.05). Moreover, the higher the CSNRT in Group A pts, the more severe the ECG abnormalities of SSS. 2) Pts without PM, both of Group A and Group B, noted during the follow-up period a disappearance of neurological symptoms (syncopes and/or dizziness) and of ECG abnormalities of SSS in more than 50% of the cases. However, this was less evident in Group A pts compared with Group B pts (53.8% vs 78.6% regarding neurological symptoms and 54.3% vs 74.1% regarding ECG abnormalities of SSS) as well as in pts with organic heart disease in comparison with those with primitive SSS. Moreover, the number of pts who needed PM implantation during the follow-up period due to the worsening of clinical and ECG signs of SSS were higher in Group A than in Group B (20% vs 6.5%). The occurrence of cardiac death among the pts without PM was similar in pts of Group A (8.5%) and in those of Group B (9.7%). One pt of Group A without PM died suddenly (less than 1 hour). 3) Pts who required PM implantation were older (p less than 0.01) and showed a prevalence of organic heart disease higher (p less than 0.05) than those who did not require PM implantation. Pts with PM, both of Group A and Group B, showed a complete disappearance of syncopes and a clear-cut reduction of dizziness after implantation of it. On the contrary, dyspnea nearly always persisted and sometimes appeared when initially absent. Sudden and non-sudden cardiac death in PM pts (13.6%) was somewhat more frequent than in those without PM. 4) The incidence of stable atrial fibrillation was 12.1% in pts without PM and 27.2% in pts with PM. The occurrence of stable atrial fibrillation in pts without PM was generally not followed by clinical improvement. 5) The incidence of cerebrovascular accidents was approximately 8%. The accidents always occurred in pts with organic heart disease and often in the older pts (mean age: 75.1 yrs +/- 5.7) particularly in those with PM. A bradycardia-tachycardia syndrome was observed only in 3 pts who had a stroke...