The Deans, who had scored 30 goals in winning their opening five games, were held 1-1 at Five Heads Park.
Horndean are six points clear of second-placed Moneyfields, while US remain unbeaten after five games, drawing three of them.
5. Horndean goal celebration by scorer, right. Horndean v US Portsmouth U18s, Hampshire Combination League Picture: Chris Moorhouse (111020-27)
Martin Buckland has just scored for Horndean against US Portsmouth. Picture: Chris Moorhouse Photo: Chris Moorhouse
6. Horndean goal celebration. Horndean v US Portsmouth U18s, Hampshire Combination League Picture: Chris Moorhouse (111020-26)
Martin Buckland is congratulated after scoring for Horndean against US Portsmouth. Picture: Chris Moorhouse Photo: Chris Moorhouse
7. US Portsmouth defend against a header from Harry Webb (5). Horndean v US Portsmouth U18s, Hampshire Combination League Picture: Chris Moorhouse (111020-25)
Horndean's Harry Webb (5) gets in a header at the US Portsmouth goal. Picture: Chris Moorhouse Photo: Chris Moorhouse
8. Horndean v US Portsmouth U18s, Hampshire Combination League Picture: Chris Moorhouse (111020-24)
Horndean (red) v US Portsmouth in the Under-18s Hampshire Combination East Division. Picture: Chris Moorhouse Photo: Chris Moorhouse