Understanding the circumstellar environment around young stars is of crucial importance for resolving the initial conditions prior to planet formation. There is a growing evidence of dust concentrations revolving around so called Algol-type young stars. These stars are somewhat more evolved intermediate-mass pre-main-sequence (PMS) stars thus being the predecessors of stars showing the Vega phenomenon. An important link between the PMS phase and the Vega phenomenon is the proposed cometary activity. Grinin et al. (1991) explained the Algol type optical behaviour with proto-comets while Beust et al. (1991) modelled optical spectral features with infalling comets. Prusti & Mitskevich (1994) discovered far-infrared variability in the Algol type PMS stars and linked it with dust arriving in the vicinity of the star along an elliptical orbit. Similar kind of dust replenishment mechanism may play a role in Vega which has too small grains, with respect to the age of the star, in its circumstellar environment (van der Bliek 1994). Intermediate mass stars clearly show activity in their circumstellar environment and the IRAS work by Prusti & Mitskevich (1994) has demonstrated that the activity is easily measurable with ISO. We are proposing a straightforward light curve monitoring programme for Algol type PMS stars and the three most prominent Vega type main-sequence objects. The wavelength dependence of the variation will give an access to the temperature of the dust causing the variation and the multi-wavelength amplitudes provide information about the masses involved. ISO is the only possibility in the near future to provide the crucial information about the processes in the circumstellar environment of intermediate mass stars: we should use this opportunity to collect the data and record the phenomenon.